The sand around the supports of Windang Bridge is being gradually washed away from fast-flowing waters heading out to the ocean.
Ever since the waters of Lake Illawarra were open to the ocean back in 2007, the speed of the flow under the bridge has increased.
This has seen a change in the composition of the bed under Lake Illawarra in the vicinity of the bridge supports,
The bridge sits on supports known as piles that are sunk into the sand, which means changes on the lake floor could affect its stability.
Last month a Transport for NSW barge was checking on the condition of the lake bed.
It noted sand had been removed from around the bridge supports - an action known as scour.
"An increase in the amount of scour at the northern end of Windang Bridge in Lake Illawarra has been observed over the last two to three years," a Transport for NSW spokeswoman said.
"The last underwater inspection carried out in May and June 2019 confirmed this."
The Transport for NSW spokeswoman said measures will soon be taken to deal with the issue.
"Rehabilitation work is planned later this year to protect the piles against further erosion caused by the permanent lake opening," she said.