This follows deep cracking of the road pavement as a result of rock and soil slippage, caused by record rainfall.

The issue is unrelated to the major work that is going on to duplicate the Woronora River Bridge, which has required full closure of the same section of road on some weekends and weeknights.

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Transport for NSW Chief Operations Officer Howard Collins said "unprecedented" wet weather had caused land movement and damage to several areas along Heathcote Road.

"Cracking caused by water-related ground movement following record rainfall was detected in the westbound lane of Heathcote Road east of the Woronora River at the start of July, forcing a closure of the westbound lane," Mr Collins said.

"Since the discovery of the cracking, further movement has taken place, which indicates significantly unstable ground conditions for vehicles to travel on.

"To ensure the safety of motorists and workers, remediation works can only go ahead after the slope is found to have remained stable for two consecutive weeks, which is dependent on weather conditions and is likely to take months.

"Maintenance crews face challenging working conditions along a steep and heavily vegetated site.

"The westbound lane can only reopen once remedial works are complete and the road is safe once again."

Mr Collins said Transport for NSW would carry out maintenance work this week at five other sites along Heathcote Road, between New Illawarra Road and Princes Highway.

"The work on Heathcote Road will increase safety and seek to prevent future damage due to weather or traffic impacts," he said.

"The work will be carried out overnight to minimise disruption to road users and won't be carried out on Saturdays or public holidays."

Mr Collins said the closure of the westbound lane meant work on the bridge upgrade project could be accelerated.

"While the lane is closed for the emergency repairs, we are going to increase concrete pours, rock cutting and drilling for the new bridge," he said.

For the latest information on emergency repairs to Heathcote Road go here.