Transport Minister Dean Nalder said the State Government was committed to Roe 8, an extension of Roe Highway to Stock Road.
A road reserve was set aside in the 1950s to accommodate a transit route from there to the Fremantle Port, but the land was removed from the Metropolitan Region Scheme in 2003 by the Gallop government.
Mr Nalder said the route for the first part of the highway extension had already been identified and was a 5.2km extension of Roe Highway from Kwinana Freeway to just west of Coolbellup Avenue.
The Government was still finalising the route for the second section, which would include upgrades to Stock Road, Leach Highway, High Street and Stirling Highway, spanning 8.2km.
Mr Nalder said he had asked Main Roads to investigate a tunnel route to avoid immediate upgrades to Stock Road and Leach Highway.
�The alternative route extends Roe Highway west of Stock Road along the highway road reserve in Hamilton Hill, then heads north as a tunnel, starting just before Clontarf Hill and running underground to Stirling Highway near High Street,� he said.
He said three companies had been hired to assess the feasibility of the alternative route and they would report back to Main Roads in about three months.
The Federal Government has committed $925 million to the project and the State Government is contributing $650 million.