Picture the scene. In an airy office in Melbourne, four people are teasing one another mercilessly about their sexual habits. One of the two women has let slip that she thinks there is a preordained order to mating. The incredulous men are questioning her closely about what this means.

This conversation took place in a script meeting for the TV series The Secret Life of Us. As anyone who has been following this joyously murky relationship drama will know, it was lifted almost directly and put in the mouths of the characters to great effect. It became the great debate over the order of sex.

This was just one of many highlights in the opening episodes of this charmingly addictive Australian ensemble piece, which has had critics raving about its classy scripts, fine acting and irrepressible sense of cool.

For fortysomething John Edwards, the fights between him, his female co-producer, Amanda Higgs, and writers, Judi McCrossin and Christopher Lee, are why Secret Life has been such a success. 'The show is about the minutiae of life, the clash between a male and a female perspective and those kinds of very natural tensions. We kept on fighting and we kept on saying this is the show, this is what we have to put into the show. Anything essentially emotional just got dragged into the centre of the drama.'

For anyone who has missed the first nine episodes on Channel 4, Secret Life tells the story of nine mismatched twentysomething friends in Melbourne, eight of whom share the same building. There's Evan, the struggling author; Alex, the successful surgeon who is hopeless in love; Kelly, the slightly overweight romantic; Jason and Gabrielle, the warring married couple; the acting couple, Miranda and Richie (though Richie may well be gay); adorable scaffolder Will and gay barman Simon.

Not as cloying as Friends or as cynical as This Life, Secret Life is smart, character-based drama that relies heavily on its sharp observations about life's dilemmas, large and small.

Edwards set out with the very simple idea of making a show about moral ambiguity. 'Most Australian shows had been about a community and every week along comes evil and every week your heroes deal with evil, chuck it out and on you go with life. We set out to do the opposite, to see how people existed in life's grey areas.'

It obviously helps ratings that the show is stocked with mostly beautiful young things but the idea wasn't to make drama exclusively for young people. 'We set out to tell the truth,' says Edwards, 'And it just so happens that in your mid-twenties is the time when you are at your most morally ambiguous. I'm in my forties and I've got to tell you, these issues don't go away. The locales may change a bit and sometimes you've got more money but the issues are the same.'

For writer Judi McCrossin, whom Edwards describes as 'a great blend of the characters of Alex and Kelly', the important thing was to highlight life's confusion. 'We didn't want to have the sorts of characters that were so eloquent you think, "Why have these people got problems? They can articulate every feeling they've ever had". Often, you have feelings that you have no idea about. I wanted to give a voice to that bewilderment.'

In her mid-thirties, McCrossin based the characters on friends and acquaintances and hopes that people will appreciate the show for its sense of reality. She had most fun with the creation of Evan, her own idea of a fantasy boyfriend were she still in her twenties. 'You'd dream of having a boyfriend like that, who was cute, funny and nonchalant and said witty things. It allowed me to put all of that angst of my twenties into the perfect guy, even though he's not so perfect, but he's kind of adorable.'

Beautiful, confused, anally retentive Alex is played by Claudia Karvan. At first, she says: 'I was little bit baffled by her and was wondering why she was being so completely stupid. She was so on top of it in one side of her life and so upside down in another. I got it in the end, it was just surprising to see someone so flawed on television.'

Alex's biggest storyline was sleeping with her best friend's boyfriend and losing her friend. This caused some problems for McCrossin, who had told all and sundry that Alex was based on her. 'My best friend rang me up because she'd just seen the bit where Gabrielle tells Alex, "I loved Jason, but I really loved you". And she said, "I just imagined us not being friends any more" and started crying. But her husband said to me, "You know how you think you're a bit like the Alex character? When are you planning to have sex with me?" Can you imagine? But I suppose at least it had struck a chord.'

It's a seductive world they've created, set against a backdrop of gorgeous colours, reliably seedy nightlife and the certainty of having at least a couple of friends to fall back on. You worry with the characters in a mild way and smile knowingly at their worst excesses and their conversations about ice-cream headaches, but you know that nothing really bad will happen.

'We didn't set out to make an optimistic show,' Edwards laughs. 'It was just a natural way for us to tell these stories. In telling stories about the greys we end up telling it in a colourful way. You could do worse than that.'

• The Secret Life of Us continues on Wednesdays on C4 at 10.55pm