A bridge at risk of collapsing on Main Road 92 between Nowra and Braidwood is to cost almost half a million dollars to replace.

The timber bridge crosses St Omers Creek west of Nerriga, and so great is the risk of catastrophic failure, Palarng Council has been forced to install emergency supports underneath the structure.

At the same time traffic on the bridge has been reduced to one way, with portable signals installed to regulate the flow of vehicles.

Council estimates the cost of installing new high level concrete culverts will be around $450,000 and given the large outlay, is enable to estimate when work will start.

Traffic counts on the road have doubled since the upgrade was competed between Nowra and Nerriga a couple of years ago.

Engineers believe the amount of traffic on the bridge has accelerated it's deterioration.