The State Government has been told it needs to build an 80m sea wall in Coalcliff to protect land just north of the bridge from coastal erosion threatening Lawrence Hargrave Dr.

Specialist geotechnical engineers have warned Roads and Maritime Services (RMS) that it needs to act quickly to stabilise the land or risk damage to the road and expensive and disruptive engineering works in the future.

The affected area is beneath the roadway in an area known as the ‘‘northern amphitheatre’’ and has been sinking closer to the sea as wet weather loosens the earth, sending boulders rolling down to the water.

Large waves have washed away many of the boulders, which has contributed to the landslip.

Internal government engineering reports, obtained by the Mercury under freedom of information laws, reveal the extent of the problem.

The area is particularly vulnerable to landslip, with some areas having moved 1.5m since 2006.

RMS has three options to stabilise the area, all involving the construction of a sea wall.

However, the longer the department waits, the more expensive and precarious the operation becomes.

Building a wall requires construction crews to work on a narrow stretch of land wedged between a sloping land mass and the sea.

The longer it takes, the harder it becomes for workers to reach the location safely as the area slips closer to the ocean.

If the Government waits too long, equipment will need to be brought in to secure the site to the cliff side before work can begin, forcing the extended closure of Lawrence Hargrave Dr.

In the reports, geotechnical engineers warn of potential damage to the road in the event of a further slip.

‘‘The risk of damage to property and consequential effects, assuming a deep-seated instability, was assessed as ARL1 - the highest level of risk,’’ one report, by geotechnical engineers AECOM, said.

The company declined to comment when contacted yesterday.

An RMS spokeswoman said there were no homes affected by the issue and that a final decision on a long-term solution had yet to be made.

A short-term solution to stop the slippage has been approved and will involve putting in place boulders which will help buttress the land. Funding for a long-term fix has still not been decided.

‘‘RMS will be starting shorter-term repairs in the coming weeks and will finalise a long-term management plan for the area later in 2012,’’ the spokeswoman said.

Roads Minister Duncan Gay’s spokesman said while the erosion was not affecting the road at the moment, it would eventually cause problems if left untreated.

The spokesman said the area was not believed to have been an issue at the time Sea Cliff Bridge was built in 2003-2004.

The bridge, opened in 2005, was designed to avoid rock falls, but not land slippage below the roadway.

‘‘The northern amphitheatre area was assessed during the planning of Sea Cliff Bridge, which opened in 2005. At the time the main concern was material falling from above the road not slips below,’’ the spokesman said.

‘‘There was no significant movement in the embankment below the road until 2008.’’

He said short-term repairs would be carefully planned to ensure minimal disruption, but added: ‘‘The traffic impact of the permanent work cannot be determined until the solution has been finalised.’’

The Illawarra has had one if its wettest summers on record, causing further problems. The weather has already contributed to one landslip in the northern amphitheatre, forcing RMS to put in extra monitoring equipment last week.