The Illawarra Highway will be closed at Macquarie Pass between 9am and 3pm on Thursday and Friday for emergency road work.
Repairs need to be made to the road after a slip.
Drivers will be detoured via Picton Road while work is carried out to replace blocked drains next to the slip, adding around 20 to 25 minutes travel time.
Roads and Maritime Services has started preliminary work to repair the slip which occurred after heavy rain at the end of November.
The RMS said the road needed to be closed for the safety of road users while heavy machinery is used.
A contra flow will be in place while the northbound carriageway is closed to stabilise the slip and clear vegetation ahead of the full closure.
The highway will be open to traffic outside working hours.
Electronic message signs are in place to advise of changed traffic conditions and the detour via Picton Road.