Transport for NSW executive director regional and outer metropolitan Roy Wakelin-King said the bridges played an important role in the economic development of regional NSW.

"These bridges were ahead of their time when they were built, showcasing leading engineering practices, and connecting local communities to one another," he explained.

"Transport for NSW has worked closely with the Heritage Council of NSW to ensure those bridges with strong local ties remain protected and restored as necessary.

"We've reviewed the conservation list of bridges developed in 2012, with eight - Beckers bridge which crosses Webbers Creek at Dunns Crossing at Mount Thorley in Gresford; Bulga bridge which crosses Wollombi Brook on Putty Road at Singleton; Junction bridge on Rouchel Road near Scone; and Cooreei bridge which crosses the Williams River on Stroud Hill Road at Dungog - being added, while seven which did not meet the strategy objectives were removed.

"Conservation will ensure the retained bridges continue to provide safe access for all users, including pedestrians, cyclists, motorists, agricultural vehicles and freight."

Of more than 400 timber truss bridges built between 1860 and 1936, 48 remain, with 39 managed by Transport for NSW.

There are five different types of truss bridges on the list, varying in style, length and truss configuration, with a total of 27 to be conserved, up from 26.

"The order of bridges to be replaced will be determined by the condition of the bridge and the road operation requirements," Mr Wakelin-King said.

"The community will be kept informed throughout the process.

"All bridges for retention will be upgraded and strengthened to support the road network without compromising their state heritage values."

Those bridges removed from the list will be progressively replaced and removed with appropriate heritage interpretation signage to be installed at these locations to provide a historical record.

Additional timber truss bridges to be included under the conservation strategy:

* Abercrombie bridge which crosses the Abercrombie River on Goulburn Road, south of Bathurst;

* Beckers bridge which crosses Webbers Creek at Dunns Crossing at Mount Thorley in Gresford;

* Bulga bridge which crosses Wollombi Brook on Putty Road at Singleton;

* Cooreei bridge which crosses the Williams River on Stroud Hill Road at Dungog;

* Crankies bridge which crosses the Coolumbooka River on Cathcart Road at Crankies Plains;

* Gillies bridge which crosses Black Creek on Wilderness Road near Rothbury;

* Junction bridge on Rouchel Road near Scone;

* Tooleybuc bridge which crosses the Murray River on Tooleybuc Road at Tooleybuc