The $9.1 million project includes the demolition of the old timber bridge and construction of the new 20m steel and concrete bridge catering to a higher load capacity.
The project also includes construction of a second lane eastbound on the bridge to separate traffic turning left and right on to Whatley Crescent, realignment of the cycle and pedestrian path through a 30m underpass and a new footpath across the bridge.
Transport Minister Dean Nalder said replacing the century-old bridge with a contemporary structure would ensure it met current safety standards and improve traffic flow during peak periods.
Mt Lawley MLA Michael Sutherland said an update was long overdue with the bridge built in the era of light motor cars and horse and cart.
�The extra lane and upgraded bridge will greatly assist in alleviating congestion, more particularly during peak hours, and improve traffic flow,� he said.
The new bridge is the final stage of a $22 million State Government project to improve safety and efficiency on the local road and rail network with the Seventh Avenue bridge opened in May.
Traffic will be detoured over the railway via the Caledonian Avenue level crossing and the new Seventh Avenue Bridge with Main Roads encouraging road users to use Guildford Road and Beaufort Street as alternate routes.
The project will take about eight months to complete and will create 15 local jobs.
The Third Avenue bridge carries about 10,000 cars per weekday.