A sweeping four-lane bridge across Hobart's Derwent River will cost $535 million and represent Tasmania's most significant transport infrastructure project in more than half a century, the state government says.

The city’s northern Bridgewater Bridge has long been marked for replacement and state infrastructure minister Rene Hidding unveiled the plans, with construction set to start in 2019-20.

“This will be the most important single transport infrastructure project in the state since (Hobart’s main) Tasman Bridge was built in the 1960s,” the minister said.

Canberra has already indicated a willingness to back the project and the Tasmanian government will now seek financial backing from the Commonwealth.

“The current Bridgewater Bridge was built in the 1940s and requires replacement as it places height, mass and width restrictions on vehicles,” Mr Hidding said.

The current bridge carries an average 18,500 vehicles per day and the new version will add lanes, while offering pedestrian and cycling facilities, with a 16-metre clearance above the river.