Photos: Traffic flows on new Heathcote Road bridge over Woronora River at Engadine

The old bridge will be closed for about three months while it is upgraded.

When that work is completed, westbound traffic will use the new bridge and eastbound traffic the old bridge.

For more than a year, the state government argued that duplicating the old narrow bridge wasn't feasible financially or from an engineering perspective.

Built by the military in 1943, the bridge had one lane in each direction and was the scene of many serious accidents.

Transport bureaucrats forged ahead with plans to widen the bridge and approaches, still with only one lane in each direction.

But, the community and Sutherland Shire Council, with the support of Liverpool and Wollongong councils, strongly opposed the plans.

A few days before Christmas 2021, the government bowed to the pressure and announced a second bridge would be built alongside.

Minister for Metropolitan Roads Natalie Ward and Heathcote MP Lee Evans were at the new bridge on opening day.

Ms Ward said journeys through the southern parts of Sydney would be transformed by the project.

"The $73 million infrastructure upgrade is the critical link between Greater Sydney and Wollongong, with 22,000 motorists using it every day," she said.

"The Liberal and Nationals Government has listened to the Heathcote community and delivered this new bridge that will provide commuters more reliable and safer journeys.

"This project was finished on time and on budget despite 60-plus wet weather days and eight flood events during construction."

Mr Evans, who fought the bureaucrats in an endeavour to have the bridge duplicated, said he could hardly believe his eyes.

"It has been a long, hard battle," he said. "I feel like an enormous weight has been lifted off my shoulders."

Mr Evans said the new bridge would operate with one lane in each direction while maintenance work was carried out on the old bridge.

"A reduced speed limit of 60km/h will be in place while both lanes of traffic are temporarily using the new bridge," he said.

"I thank the community for its continued patience while this complex and critical project is being delivered."