Roadworks at Gerringong and South Nowra will eventually help ease some of the chronic traffic problems experienced on the Princes Hwy, particularly in peak holiday times when the Shoalhaven's population swells from 97,000 to about 320,000 from Boxing Day.

However the South Nowra upgrade in particular is already causing significant delays for weekend traffic.

Shoalhaven Mayor Joanna Gash was caught in a three-kilometre jam recently. She said although the roadworks were not a council project, she intended to raise the issue with council staff to see if there was anything they could do this summer, given there were few alternatives around the area.

"It is an inconvenience, but at least something is being done," Mrs Gash said.

"It is frustrating ... but people need to think of the long-term benefits of it all."

This week, Roads and Maritime Services (RMS) advised motorists of changed traffic conditions for work on the Gerringong upgrade.

Work to install bridge supports opposite Belinda Street and at Crooked River started yesterday and will progress to Rose Valley Road near Omega Lane, and possibly Toolijooa Road, before the end of the year.

Work will be carried out on weekdays from 7am to 6pm and from 8am to 1pm on Saturdays, provided the weather permits. Work to install the bridge support structures will include the removal of soil, the installation of a steel reinforcement cage, and then filling the hole with concrete.

RMS advises motorists to plan their journeys, allow extra travel time, and obey all signs.

The $310 million Gerringong upgrade will provide 7.5 kilometres of upgraded highway and includes two new interchanges.

At South Nowra, the $62 million upgrade covers 6.3 kilometres of the Princes Highway.