Kiama Council has closed Jamberoo Mountain Road following a large landslip caused by heavy rain overnight.
Council workers have installed safety barriers at the bottom and top of Jamberoo Mountain.
The road is closed between Illawarra Highway, Robertson and Daltons Road.
Motorists need to use alternative routes such as Macquarie Pass or Princes Highway.
Access for local residents is possible up to Misty's Lane, and down to Barren Grounds.
The road is expected to be closed for at least the rest of the week while the damage is assessed.
Repairs could take some weeks to complete, which will prohibit the road from being opened.