Components of the new $274 million-dollar concrete bridge over the river have been taking shape off-site, earthworks have been shaped at the southern approach, and the support piling work has progressed.

The new 425-metre-long bridge is being constructed 40 metres to the west of the present two-lane bridge, which for 63 years has used a lifting counterweighted span to allow tall marine traffic to pass through twice a day.

The current steel bridge was opened in 1956, replacing a vehicle punt across the river. Significant parts of the major construction project have stayed in the local area, with the bridge's 168 concrete box sections built at a temporary facility six kilometres south of Mogo.

The new four-lane bridge, with its separate three-metre-wide cycleway and pedestrian walkway alongside, will be supported by concrete piles, with piers located on top.

The piers will support the deck sections, which will be lifted in place using cranes on barges and tied together using steel bars and stressing cables.

The bridge will be 12 metres in height from the water at high tide and is scheduled to be completed in 2023.

New turning lanes are planned for the roundabout at the eastern end of the Kings Highway intersection with the Princes Highway, to ease the significant congestion which occurs during holiday periods as dense traffic from the west and north converges on the one roundabout.

As a further congestion-buster during those peak periods, "seasonal" traffic lights will be installed at the roundabout.

Meanwhile, preliminary work has also begun on a replacement for the 55-year-old two-lane Nelligen Bridge, with $9 million set aside by the NSW government for design and feasibility.

Although tenders closed last year, a change to the design was required, which delayed the project. No update has been provided on when major work at Nelligen will begin.