Work to reduce the risk of collapse of Windang Bridge will start next week.
Transport for NSW is concerned about the stability of the bridge piers, which are sunk into the sand on the seabed.
Water flowing past the piers is washing away the sand around them - a process known as scour.
To counter that a barge in Lake Illawarra will place rocks around the piers.
"It will also ensure the bridge is safe and operational for motorists, and extend the life of the bridge," a Transport for NSW spokeswoman said.
"This work will be carried out at the north end of Windang Bridge and take around three months to complete."
Work will be carried out between 7am and 5pm on weekdays and 7am to 1pm on Saturdays.
The TfNSW spokeswoman advised people should avoid fishing around the worksite.
A site office will be set up at the southern end of the car park off Fern Street, taking around 14 car spaces.
Additional truck movements, barges and excavators will be required in Fern Street throughout the project to allow for the delivery of rock stockpiles to the temporary site offices.
There will be no impact to traffic on the bridge or to the shared paths for bicycles and pedestrians.