There will even be police patrols there over summer with offenders warned they will be fined.

NSW National Parks and Wildlife (NPWS) says there is "no access" to the base of either Gerringong Falls or Belmore Falls, but that hasn't stopped people making the risky bushwalks there for a swim and a selfie.

Instagram posts document travellers' adventures, showing how the base of Gerringong Falls is beautiful to the eye - but a potentially deadly bushbash of several hours each way.

Some of the accounts purport to be travel companies or promoters, including advertising guided walks to Gerringong or "5 things to do" when you're there.

Another website, with an eye to helping Britishers avoid getting killed in Australia, warns that you "absolutely shouldn't do this hike", describing it as a "death trap waiting to happen".

You shouldn't be there at all, is the message from NSW National Parks and Wildlife Area Manager Graham Bush.

He said a recent surge in illegal access, fuelled by social media, could be a matter of life or death.

"There is no bushwalking access to the base of Belmore and Gerringong Falls," Mr Bush said.

"These areas are dangerous and there is a high risk of injury or death from unstable rock faces, dangerous cliff edges and slippery surfaces.

"The hazards are very real to both the public and emergency service workers who risk their lives responding to people who have slipped, sadly often fatally.

"Signs warning of the dangers and prohibiting access are installed at both waterfalls, however visitors continue to walk past and ignore these warnings and attempt to access the base of the falls.

"Those wanting to cool down this summer are best to head to a patrolled beach or other nearby waterways such as the Shoalhaven River.

"Visitors can still walk to the top of the falls and take in the spectacular views, but we don't want one more person to lose their life for the sake of a selfie," Mr Bush said."

NPWS said its rangers and police will be conducting patrols in the Belmore and Gerringong Falls areas and offenders will be fined.