Windang Bridge needs more work to make sure it stays standing

Major work on the bridge piers - which are sunk into sand on the lake bed - began in 2019.

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Water flowing past the piers had been washing away the sand around them - a process known as scour.

The result was a risk that the stability of the piers could be compromised.

That work in 2019 included placing tonnes of rock around the base of the piers.

February next year will see the start of an 18-month project to place a further 16,000 tonnes of rock under the bridge.

"This will provide a stable support for the whole bridge in the future without impeding the flow of water through the Lake Illawarra entrance channel," a Transport for NSW spokesperson said.

Following the 2019 operation, Transport for NSW continued to monitor the Windang Bridge and kept the work compound at the Fern Street boat ramp car park - where rocks were stored - to ensure a quick response if urgent repair work was required.

""Hydraulic modelling and survey of the lake entrance channel and bridge piers has been carried out on a regular basis since the completion of earlier work," the spokesperson said.

"Survey and modelling data has been used to develop detailed designs for this additional protection of the bridge from future scour."

The set navigational channel with two spans of the bridge marked in colours outlining the path for boats will not be blocked while the work is undertaken.

The entrance will remain open to boats, however, there will be certain areas under the bridge where boats cannot go.