A website designed to help people have extra-marital affairs arrives in New Zealand this month.

Would-be adulterers can sign up to www.ashleymadison.com for free and create a profile to let people know what they look like and what they want.

But they must buy credits to communicate with other users to arrange illicit liaisons.

Creator Noel Biderman said he brought the service here to capitalise on Kiwi interest in the site's Australian launch last week.

"Four to five thousand people were trying to log in from New Zealand. We thought while we were (in Australia) we should grab that opportunity."

The site, which uses the slogan "Life is short, have an affair" started in his native Canada in 2001 before expanding to the US and UK.

It claims to be the world's "premier discreet dating service" with 5.7 million anonymous members.

Biderman said he had earned "tonnes of millions" from it but refused to reveal exactly how much.

He said most users were men looking for sex rather than new relationships, but denied the site encouraged people to cheat. "I'm not going to generate infidelity - if it's going to happen we want people to do it in a secure place.

"It's a social networking site where people can be honest with each other. The reason I built the service was because 30 per cent of people on singles websites are not single at all. Most of those would prefer a network of their own."

The 38-year-old former lawyer and sports agent said he was a "happily married" father of two and would be "devastated" if his wife cheated on him.

"But there is no way I would blame it on an inanimate object like a website or say it was the hotel room or the cellphone company's fault.

"I wouldn't even blame my wife or her lover - I'd look in the mirror and ask myself, 'How did I fail in my relationship?"'

Auckland psychologist Nathan Gaunt said people who signed up had to be prepared for repercussions.

"People have to question why they are doing it. It's a good time for people to take stock of the relationship if they are considering that sort of thing."

Sex expert Blair Bishop said the site was a "tangible and realistic acknowledgment" of a change in society - traditional monogamy was a "myth" and infidelity was a part of life.

"Infidelity can be damaging ... but not always. I suspect more people want to do it, but it's not something we talk about."

Gaunt said any affair was laced with "deception, deceit and betrayal".

"Society may have changed and relationships may have changed but I don't think the need for honesty and trust has changed. If anything, they have become more important."

Biderman said the site was constantly monitored and profiles containing aggressive or inappropriate content were removed.

But it was impossible to stop sexual predators trawling the site and users had to rely on common sense to stay safe.

"We have been operating for almost nine years, servicing more than 5.5 million members, and have never had a single incident."

A controversial reputation
* Noel Biderman's site has generated so much controversy he was flanked by two security guards when he attended the Australian launch last week.

* Users have accused him of ruining relationships. He has been the topic of heated debates on US talk shows including Dr Phil, Ellen and Larry King.

* In February he offered to pay the US city of Phoenix $10 million to rename its international airport the Ashley Madison International Airport for five years. It said no.

* Two months earlier he offered to subsidise public transport fares in Toronto if the transit authority allowed him to advertise on its property. It too declined.

* US TV network NBC refused to show ads for the website during coverage of the 2009 Superbowl because they were too racy.