A collapsing housing development on Bulli Pass is being investigated by Safe Work NSW after rain caused the steep land to be unstable - more than a year after warnings were given.

Pictures show the extent of the damage at the site, where work has stopped since the collapse last week.

A tree has been pushed through a wall in one place, causing the wall to collapse, while slipping land has destroyed another wall.

This comes after neighbours had been raising concerns about the excavation for several months.

Neighbour Wendy Ristuccia, has previously documented cracks in the concrete pool surrounds which she said were not there beforehand, and were getting worse.

Ms Ristuccia, a prominent real estate agent in the area, had earlier raised her concerns with Wollongong City Council. She had been told the site was under the auspices of a private certifier, and she may have to take action in the courts, but the council also took some action about the instability.

"That land has been slipping for maybe the last three years and nothing has been done about it," Ms Ristuccia said.

"My pool has progressively started moving ... I have got measurements and pictures over time.

"The expansion joint is getting bigger where my fence was connected on the back deck - that has come away from my fence.

"My main concern is I want it stabilised and fixed."

Her photos show concrete level slippage of up to 40mm, cracks widened about the same distance and a gate latch that no longer meets properly.

Ms Ristuccia sought legal advice, and also took her case to Member for Heathcote Maryanne Stuart.

After representations were made Wollongong City Council wrote back to Ms Stuart, saying they had organised a site meeting where the builder was told to do a geotechnical report and perform the mitigation works it recommended.

The council said staff had been "investigating" the works on this site since August 2022.

"As part of the investigation, council has recently requested that a geotechnical report be prepared by the builder commenting on the structural integrity of the slope created by the excavation," its letter stated.

"Council will now work with the builder to ensure that the recommendations detailed within the report are complied with in a timely manner."

It said Council's Regulation and Enforcement staff would monitor the site and take further action if mitigation works were not completed.

All of this was before December, when the landslip which confirmed neighbours' worst fears.

It's understood there have been multiple changes of builder, and the current builder, MGM Construction & Development, is not necessarily responsible for previous excavation work.

The Mercury has tried contacting MGM head Michael Mikhail but has not heard back.

The private certifier, Building Certifiers Pty Ltd, referred questions to the builder.

A spokesman for Safe Work Australia said an investigation was underway.

"SafeWork NSW is aware of an incident involving the failure and collapse of an excavation and adjacent land at Thirroul," he said.

"An investigation is currently underway and as such no direct comment is available at this time.

"SafeWork NSW reminds all builders and trades involved in excavation works to ensure the stability of excavations and sloping land.

"Where required, we encourage the use of geotechnical engineers for expert advice on ground retention systems."

The development is being marketed by A-List Real Estate as "embracing its location and luxury identity within one of Illawarra's most sort after (sic) residential postcodes".

It would have the quality features "that you would expect from a boutique development in the northern suburbs".