The website also identified the "cheating hotspots" in New Zealand, charting where its 54,000 members live to create a "cheaters per capita" table.
The distribution was fairly evenly spread - the website said 1.26 per cent of Christchurch people were signed up, making it the top hotspot , while Auckland and Hamilton were second with 1.23 per cent of their populations on the site.
Wellington (1.22 per cent) and Dunedin (1.2 per cent) rounded out the top five.
As well as coming second for the largest number of members, the City of Sails was also found to have the highest proportion of females hooking up outside their marriage.
Thirty-six per cent of Auckland members were women, 7 per cent higher than the national average of 29 per cent.
The founder of AshleyMadison.com, Noel Biderman, said the study demonstrated that "infidelity is part of human nature".
"It transcends geography, age, political disposition and religion."
The site, which uses the slogan "Life is short, have an affair", was launched in New Zealand in April 2010.
It started in Mr Biderman's native Canada in 2001 before expanding to the United States and Britain. Membership of the website doubled last year.
- staff reporter