The Perth Link Roads Project is the biggest part of the $500 million 10-Year Midland Highway Action Plan.

Since the start of April, traffic has been diverted to a temporary side detour with a reduced speed limit while a longer-term detour is constructed.

From May, traffic will be transferred to the longer-term detour through a newly constructed section of the site, a Department of State Growth spokesman said.

"This detour will be in place for much of the life of the project to allow work to proceed smoothly and to ensure the safety of road users and construction workers," he said. "Drivers will get a close view of the two new bridges as they drive underneath the new structures."

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Works will continue on the construction of the four-and-half kilometre highway around the western side of Perth. This includes a new interchange at Illawarra Road and a roundabout near the South Esk River Bridge.

"The project will also include highway access ramps at the northern end of Perth, an underpass for the existing rail line south of Illawarra Road and cycling path from Drummond Street to Illawarra Road," the spokesman said.

The whole upgrade is expected to cost more than $92 million and is on track to be completed as planned by 2021.

When complete, the project will provide the final link in a divided dual carriageway along the Midland Highway between Launceston and south of Perth.