The three lanes from the freeway under Roe Highway have been extended and vehicles entering from Roe Highway now have a dedicated lane.
More than 52,000 cars use the stretch daily, with that figure expected to climb to 75,000 vehicles by 2021.
The widening was stage one of a $77.5 million upgrade, with State Transport Minister Dean Nalder saying work to widen 3km of freeway between Armadale Road and Russell Road to three lanes should start next month.
South Perth contractor Brierty was awarded the contract earlier this month. Construction will finish mid-2016.
Cockburn Mayor Logan Howlett said the city was looking forward to the completion of the works up to Russell Road as soon as possible.