The National Parks and Wildlife Service (NPWS) has ordered a geotechnical engineer to assess the impact of the fall on the mountain's southern side, which has significantly expanded damage left by a slip in 2007.

The NPWS was told of the incident yesterday, but Mark Luchetti, who used the cliff face as training during his 14 years in NSW Police Rescue, thinks it probably happened on Thursday night or early Friday.

"I look up at that mountain every day and I first noticed it [the landslide] on Friday evening and thought 'wow'," Mr Luchetti said. "Some of the boulders would be huge, it's carved a much bigger swathe below the cliff face than the previous one.

"I'd be horrified to think anybody would be trying to walk over it or be anywhere near it because it would be incredibly unstable."

NPWS acting area manager Chris Keyzer said no track or road closures were deemed necessary.

"The amount of land lost is difficult to measure but we estimate it may be approximately 50 per cent of the volume of the 2007 slip," he said.

Asked if any work was needed to stabilise the area, Mr Keyzer said: "We have ordered a reassessment of the landslip site by a qualified geotechnical engineer and once we have their recommendations we will be able to answer this question".

On Anzac Day in 2007 it was a 1500-tonne landslide that hit Mt Keira's southern face.

At Mt Kembla, Harry Graham Dr was closed for 2? years after several hundred tonnes of rock fell onto the road in June 2007.

In September 2010, heavy overnight rain in the northern suburbs caused part of the escarpment above Wombarra and Scarborough to slide.

In 1998, a geotechnical team identified more than 190 potential landslip sites in Wollongong, including around Mt Kembla, Mt Keira, Balgownie, Wombarra, Scarborough and along the northern suburbs sea cliffs.