SYDNEY - A rock formation that is a popular tourist attraction south of Sydney is in danger of collapse without warning.

Wedding Cake Rock, which is 50m above the sea in the Royal National Park, could collapse into the ocean because it has become severely undermined, according to the New South Wales National Parks and Wildlife Service (NPWS).

Last year, a French university student that local media named as Fabien Ardoin died after falling from the rock. An increasing number of pictures of visitors performing stunts such as handstands on its edge have been shared on social media over the past three months.

It was named Wedding Cake Rock because of its resemblance to the squared silky layers of a white wedding cake. Ironically the colour is an indication that the iron in the sandstone had been leached out over time, leaving it pure white but also significantly weakened.

NPWS regional manager Gary Dunnett said safety concerns for the many visitors to the site, 30km south of Sydney, had meant blocking access to the rock shelf until geotechnical investigations were completed, which included a photographic survey of the underlying rock strata using a drone.

"The report is clear - standing on the rock platform risks a truly tragic outcome," Mr Dunnett said. He said the drone footage clearly showed that the rock is precariously balancing on the edge of the cliff and severely undercut. The geotechnical report found that the edges or the entire formation could collapse at any time.

"We will now take action to create a new and safer way to allow visitors to take in the beauty of the site without compromising their safety," Mr Dunnett said.

This is likely to mean building a new viewing platform set back from the edge, that will allow visitors to safely take shots of the rock formation. The NPWS said that about 2,000 people a month usually walk the coast track which passes the site.

But it said that number has risen to about 10,000 visitors making the trip to Wedding Cake Rock in the past few months, apparently drawn by the social media picture craze.