Australian couples are among the most faithful in the world, according to new relationship figures showing just three per cent have cheated on their loved one in the past year.

But other sex researchers argue that the major study dramatically underestimates infidelities over the lifetime of a relationship, and put the figure closer to 60 per cent.

The telephone survey of about 6,500 adult Australians found that just one per cent of women and four per cent of men in exclusive relationships had had sex with someone else in the past year.

"Australians have an impressively high level of commitment in relationships, probably one of the best in the world," said study leader Dr Juliet Richters, a sex researcher at the University of NSW.

"And it shows that Australians are certainly consistent because they practise what they preach. They overwhelmingly say that having sex outside a relationships is wrong and that's obviously what they do."

Dr Richters presented the findings of the Australian Longitudinal Study of Health and Relationships at the Australasian Sexual Health Conference in Perth today.

But the statistics were disputed by another Sydney sexual health expert and author, Dr Rosie King, who said the picture over several years or decades would be "radically different".

"The rate for cheating is more likely to be 40 to 60 per cent in any one relationship, which is depressingly high," Dr King said.

She said that historically men were more likely to cheat, and chose single women for their infidelities, but these days the sexes had almost drawn equal as women joined the workplace and gained the freedom to be unfaithful.

Dr King said remaining monogamous was a struggle for the majority of couples.

"There's no easy way to be faithful," she said.

"Every person, no matter how much they love their partner, is at times going to be attracted to somebody else.

"You can't avoid that. But what you can avoid is fostering that relationship and furthering it.

"That's a conscious decision."

AAP