Two years ago, the band hailed as Australia's Strokes went into meltdown. Even industrial quantities of dope couldn't explain frontman Craig Nicholls's erratic and violent behaviour. Then a diagnosis of Asperger's put the singer on the road to a triumphant new album.

The end, when it came, was horrible. And it had started so well. It is spring 2004. The Vines, one of the most wildly acclaimed bands of recent years, are touring in support of their second album, Winning Days. Their first, 2002's Highly Evolved, has sold 1.5m copies. Part 'Australian Strokes', part 'Antipodean Nirvana', they have quickly become one of the hottest bands in the world. Massive in America, huge in the UK.

Singer and songwriter Craig Nicholls is a proper star, out-there and bonkers. He's a prodigious and ostentatious marijuana smoker who survives on a burger-only diet. He's occasionally been rude and uncommunicative in interviews and pulled funny faces in photographs. But so what? Nicholls's wild-eyed guitar-smashing antics have afforded him iconic status among a new generation of young music fans. Rock'n'roll!

In Manchester in 2002 the Vines were the first live band seen by a kid called Alex Turner. Watching Nicholls careen round the stage like a man possessed - or just really, really stoned - the future Arctic Monkeys frontman was impressed. 'I thought, "That's what being a singer is all about,"' Turner tells me. 'When we play, I'll do what Craig Nicholls does, be all spaced out.'

But within the Vines camp it was becoming increasingly apparent that there was a dark side to being all spaced out.

Now, after beginning 2004 with a gruelling US tour with fellow Australian rockers Jet, the Vines are in Japan. Their manager isn't there but later sees footage of one of the shows. 'It's almost unwatchable,' Andy Kelly grimaces.

In the video, Nicholls is screaming at the crowd, abusing them. No mean feat - or a particularly mean feat - in a country where rock audiences are bewilderingly polite. An exasperated Patrick Matthews, the Vines's bass player and most longstanding member alongside Nicholls, walks off stage, just as he did in Boston after the singer attacked him mid-gig. Burly drummer Hamish Rosser tries to convince Matthews to come back on. Reluctantly, Matthews retrieves his instrument and he, Rosser and guitarist Ryan Griffiths prepare to start another song. Then Nicholls walks off. The gig collapses in utter chaos.

'You had to laugh,' says Kelly, shaking his head. 'Craig's kind of a genius like that. But yeah, it was pretty hard. People bend over backwards for you in Japan, but he gave his hosts a hard time.'

The Vines's touring party lurches on to the Australian leg of their world tour. The second gig is at a 450-capacity pub called the Annandale Hotel in Sydney, the city in which school kid and sometime McDonald's employee Nicholls had formed the band in 1994. The show is sponsored by national radio station Triple M and is full of competition winners and music biz types.

'Why the fuck are you laughing?' Nicholls says to the audience. 'You're all a bunch of sheep. Can you go "baa"?' He kicks out at a photographer, smashing her camera. Matthews has had enough. He leaves the stage, puts on his hoodie, then catches a cab home.

Patrick Matthews will never play with the Vines again. Triple M ban Vines songs from their station forever. The band cancel all touring commitments. The photographer goes to the police and Nicholls faces assault charges.

It is May 2004. The Vines's second album has been out for little over two months. Even their supportive UK record label is fed up with the opportunities wasted. The American label, which had already seen their erstwhile golden-boys trash the set of The David Letterman Show and be kicked off Jay Leno's show for the same, were also less than chuffed. It's all over already for this once-great band, their promise and thrill squandered by the brattish unprofessionalism, appalling manners, violent temperament and all round jerk-off attitude of Craig Nicholls. There's no excuse for behaviour like that.

"I rolled back on to the lawn and pressed my forehead to the ground again and made the noise that Father calls groaning. I make this noise when there is too much information coming into my head from the outside world. It is like when you are upset and you hold the radio against your ear and you tune it halfway between two stations so that all you get is white noise and then you turn the volume right up so that this is all you can hear and then you know you are safe because you cannot hear anything else. The policeman took hold of my arm and lifted me on to my feet. I didn't like him touching me like this. And this is when I hit him." From The Curious Incident of the Dog in the Night-Time, by Mark Haddon

Sydney, February 2006. In his office in the Vines's management's building, Andy Kelly says that no, Craig Nicholls doesn't like being touched. 'He shakes hands now though, which is great. But I'd never go and put my arm round him like I would with Ryan and Hamish.'

Shortly after the Annandale Hotel debacle, Craig Nicholls was diagnosed as having Asperger's Syndrome, the mild form of autism that the narrator had in Mark Haddon's Whitbread-winning novel.

Those with Asperger's - which is usually but not always diagnosed in childhood - find two-way conversation difficult. Keeping eye-contact is a challenge. They like routine; familiar surroundings and smells and tastes. Otherwise they can suffer from sensory overload, which can lead to bizarre motor, speech and language idiosyncrasies.

The syndrome, while at the 'high-functioning' end of the autism spectrum, explains why Nicholls - tasked with performing daily in rapidly changing, unfamiliar, uncontrolled and sensory-challenging environments - freaked out so much, so often and so alarmingly. And why he subsisted on a diet of Big Macs, bongs and cola. It was a small rock of routine in a sea of change.

'He really was in pain, and it was awful to watch,' Kelly says slowly. 'I used to sometimes think, on tour, "Are we gonna be the end of Craig? We love him and yet..."' Kelly pauses. We're killing him? 'Yeah. "Why are we making him go on tour when it clearly makes him so unhappy?" Although he would, every third show, turn in an absolutely incredible performance.' 

Asperger's also goes some way to rationalising the complete communication breakdown that could occur between Nicholls and journalists. During his very first UK interview, with NME at an American rock festival, he tried to smash the writer's tape recorder. Then locked himself in the bathroom. For 90 minutes.

Nicholls's erratic behaviour, then, ticks all the Asperger's boxes. He's not another messed-up rock star who's an accident waiting to happen. He's neuro-divergent. In other words, 'rock star' is the worst career Nicholls could have chosen.

'That's what the professor told me,' Nicholls smiles, presumably referring to his therapist.

The Vines are gathered round a circular table. The reason for their first interview in more than two years is that, against the odds - in the shadow of their creative lynchpin's mental instability - they have made a third album. Even more surprisingly, it's brilliant. Vision Valley is 31 minutes and 27 seconds of punchy power-pop. It sounds like the work of a New Wave Beatles. Their scorching first single 'Gross Out', all 77 seconds of it, is already setting fire to radios.

Hamish Rosser, 30, has the build, attire and chipper, gregarious attitude of a surf dude. Ryan Griffiths, 27, is friendly but quiet and dressed like a scruffy British indie kid. And Craig Nicholls, 28, is, well, Craig Nicholls is definitely odd. As Andy Kelly says of the youngster who began inundating him with CDs full of 'amazing songs' more than five years ago, no one who met Nicholls back then would idly wonder, 'Oh, is there something slightly different about Craig? Absolutely obviously there was.'

But the strangely boyish-man was an artist, and a herbally enhanced one at that, with a genius talent for short, sharp rock songs. He was meant to be different.

Nicholls is polite but skittish. He chain-smokes cigarettes, five in our 90-minute conversation (by some margin, apparently, the longest interview he's ever endured). His lank curtains of hair are in need of a good wash. He is overdressed for this baking Australian summer day. He doesn't take his coat off. Heavy lidded and talking as if his mouth's been numbed by the dentist, he looks and sounds doped up to the eyeballs.

It was Craig Nicholls's guitar technician, English roadcrew veteran Tony Bateman, who first suggested that rather than just being a wanker, there might be something clinically wrong with the Vines's singer. Bateman had toured with the Cure, Sisters of Mercy and Black Sabbath. He'd seen his share of rock'n'roll craziness. But as, night after night, he patched together Fender after Fender that Nicholls had smashed, he got to thinking. He downloaded some info on Asperger's from the internet and showed it to Kelly.

Nicholls's condition was revealed in Balmain Local Court in Sydney on 19 November 2004, where he faced assault charges arising from the Annandale gig. The judge dropped the charges on the condition that Nicholls sought immediate treatment. He was in therapy for six months, took Valium (but is off it now) improved his diet, and - to the amazement of all - stopped smoking marijuana. Did the diagnosis come as a relief?

'Yeah, it made a lot of sense to me,' says Nicholls in his peculiar, drawled, quasi-Australian accent. He talks haltingly. 'Just in my life and the experiences that I'd had, growing up and stuff. It was kind of a relief. It was more like an explanation.'

Given his extreme behaviour, why wasn't it diagnosed earlier? 'I wondered the same thing,' Nicholls says wryly. The looming court appearance brought things to a head, recalls Kelly. The Vines couldn't do any more gigs, and not just for reasons of their frontman's own health - 'there was a liability issue. It was just getting too dangerous. He was really terrified about the court case, terrified he was going to go to jail. He didn't want to go through that ever again. It was the start of him trying to make changes. After that he actually stopped smoking pot. Which was unimaginable - unimaginable.'

What did smoking pot do for Nicholls?

'Ahm. It just fit with my personality. I never drank alcohol. [Smoking] was just the thing I did. It made me calm. But it started not making me calm at all. It made me edgy, so that wasn't good.'

Whether cannabis use caused Nicholls's mental and emotional problems, or exacerbated existing ones, is unclear. But Robin Turner, the Vines's A&R man and long-term confidant at their UK label Heavenly, always thought his habit was a hindrance, not a help. He witnessed the toilet incident with NME in America. 'It just wasn't rational behaviour,' he recalls. 'But because Craig was smoking such massive amounts of spliff, I wondered whether his behaviour was down to a schizophrenia caused by cannabis.'

Growing up in the Sydney suburbs with his brother and two sisters, Nicholls was an introverted and borderline obsessive teenager - he escaped into art, songwriting and skateboarding. He was remote, and occasionally troublesome; his waywardness once compelled his parents to call the police (he can't remember why). Did they never take him to the doctor or suggest help? 'I remember when I was maybe 15, I spoke to a couple of psychologists. But I don't think they diagnosed me with anything. So. Maybe it grew. Maybe it became more intense when I started in the band. But I'm not really sure.' 

If you have Asperger's you can be good with numbers or words, or have focused, narrow interests. Nicholls feels 'comfortable writing songs. It's a good outlet for me. Seeing as though I'm not that social, it's a way to communicate.'

'You've definitely got a way with words,' says Rosser. 'I notice Craig has an incredible retention of lyrics and song, or even lines from comedy shows. He can see it once and have the jokes down. We're like, "Wow, how'd you remember that?"' 

Reading social situations comes less easily. 'Mind blindness' prevents easy understanding of body language, facial expressions and sarcasm.

'I think it's a mild case that I have,' says Nicholls. 'But that's not one of my strong points.'

'You've got the sarcasm down,' interjects Griffiths, to laughs all around. 'He was great at counting cards when we were playing Black Jack!' says Rosser. 'You could never beat him.' 

I give Nicholls a copy of Haddon's book. He mumbles an embarrassed thank you. He seems never to have heard of it. But Rosser has read it and tells him it's a great story. 'The kid's different to you, his Asperger's is probably a worse scenario.' Certainly, Nicholls's life doesn't seem to be governed by rules about how many red cars he sees in a row. 'But [Haddon's character] is really good with numbers, a mathematical genius,' Rosser tells Nicholls, who's handling the book gingerly, like an unexploded bomb.

'I tell you,' says Andy Kelly later, 'a year ago you wouldn't have even got to the point of giving him that book. Or him even saying thanks.'

Craig Nicholls has been off the dope for nine months now. You can hear his clear-headedness in the muscular, focused pop of Vision Valley. He's been a joy to work with, says Kelly. He can't remember details of many of his more outrageous 'stunts' - he's a complete blank on the Jay Leno Show fiasco - but when told of them is fairly contrite. He acknowledges his inner turmoil, in the opening line of the album's first song, 'Anysound': 'I am a Vine, all twisted and frayed.'

There's no pressure. Winning Days sold 600,000 copies which, as failures go, is still impressive. But the circumstances surrounding the album and tour were so disastrous and painful that there are no expectations as to how the new album will do. The fact that the band survived - that, let's be frank, Nicholls survived - to make a new album, is victory enough.

The routine of recording has suited him, and he's thrown himself into the pernickety sonic details involved in mastering the new album. Our interview only happened because Nicholls had two weeks' notice. That's how things need to be.

'The worst thing in a way, even though he has to have rest periods, is if he's got too much time on his hands,' says Andy Kelly, who clearly has as much a paternal love for Nicholls as a professional one. 'Then he overthinks things and really beats himself up about stuff. But it's a sign of how good he is now that if he rings me at midnight to talk about a song, that's fine, I don't mind taking the call 'cos I know he'll be logical about it and reasonable. We can have a two-way conversation and he'll listen. Whereas before, you used to have to walk on eggshells.'

All of which begs the million-dollar question: will - can - the Vines tour again?

Kelly winces and sighs. 'The thing we said to them a few months ago is, if they're gonna play live again, they pretty much have to be the best live band in the world. I don't mean that flippantly. If they performed disastrously, it'd be horrible - for them, for everyone around them.

'But if they were to tour again I worry that it's just gonna go back to how it was. More and more pressure, changes every day, expectations - if they're in Toronto, why can't they do this TV show? So yeah, it's a matter of managing that. It's down to us and his family and the band to keep a lid on everything so he doesn't just go the way he went before.'

What do the band think? Will they be back in the UK this spring?

'Ooh, wouldn't like to promise it,' says an eager but once-bitten Hamish Rosser.

'But we will,' says Craig Nicholls. And behind the sleepy-eyes and the greasy fringe there's something like determination.

· The Vines's new album, Vision Valley, is released on 3 April. The single, 'Gross Out', is out on 21 March