The New Zealand Aids Foundation is concerned about the number of gay and bisexual men having unprotected sex, following a surge in Syphilis cases nation-wide.

Syphilis cases have more than doubled in Auckland and Christchurch over the last 18 months.

Aids Foundation executive director Shaun Robinson said the organisation's been working with sexual health clinics in both main centres to try and stop it spreading further.

He thinks the increase is likely to be down to the uptake in the use of social media apps.

“Apps such Tinder and Grindr have made it much easier for people to find sexual partners, particularly casual sexual partners – it’s extended the sexual networking that people can do,” he explained.

Robinson said the cases are being seen in both the homosexual, and heterosexual communities.

Canterbury's Medical Officer of Health Ramon Pink said syphilis can be difficult to diagnose, so they're training GPs and nurses on how to spot it.

Pink notes that what was once a disease prevalent in the gay and bi-sexual community has now spread into the heterosexual population as well.