Relationship Services clinical leader Kirsty Robertson said she was not surprised by the results.
"That's a fairly accurate figure. A surprisingly high number of people do have an affair.
"But it doesn't necessarily mean that they don't love their partner or want to be single. It might mean that they're just not getting what they're wanting from their relationship."
Results also showed women under 30 were more likely to walk out of a relationship if their partner had an affair. Older people - particularly men over 50 - were more forgiving.
Ms Robertson said those results were also a good reflection of what she dealt with on a regular basis.
"The under-30s are really still committed to the idea of romance [and] the idea that I'm your one and only, that I'm your soulmate.
"Older people are more aware of the grey areas of the world."
The survey follows actress Gwyneth Paltrow's admission last week that she was "a romantic and a realist" with a relaxed attitude towards extra-marital affairs.
"Life is complicated and long and I know people that I respect and admire and look up to who have had extra-marital affairs," she said.
The survey revealed women were still unhappy about the division of domestic chores.
It also touches on what annoys partners in a relationship. Apparently men annoy women when they do not communicate, while men get annoyed when women are not romantic enough.
Love Affairs
* 29 per cent of Kiwis have had an affair
* 47 per cent of Kiwi couples would work through an affair and stay together
* 85 per cent imagine being with only one person for the rest of their life.
