The federal government announced in May $58 million in Nation Building 2 scheme funding and a July, 2014, commencement date for the duplication.

Acting Premier Bryan Green said the duplication would establish a four-lane highway from Perth to Breadalbane.

``Given the size and cost of the Midland Highway upgrade, it makes sense to undertake and prioritise the work, achieving the most substantial gains in terms of efficiency and safety as soon as possible,'' Mr Green said.

``It is hoped that preliminary planning work for this important project, with improved access arrangements at Breadalbane and Devon Hills Road, will begin next July.''

He said although it was not in the budget, the state government would pursue funding for a Perth bypass.

``The Perth bypass remains a long-term priority for the state government. That is why it is identified as a long-term, strategic project and endorsed by all local councils along the highway in the Midland Highway Partnership Agreement,'' Mr Green said.

``This position is in stark contrast to the Liberals who are on record as opposing a bypass of either Campbell Town or Perth.

``The Liberals' Midland Highway policy was a blatant fraud three years ago, it's even worse now.''

Opposition infrastructure spokesman Rene Hidding said the Liberals supported the current Labor plan for the highway's development.

``Following much scorn from the Labor government, the Liberals were vindicated when seven affected councils joined in a partnership agreement with the state government, which adopted the Liberal policy with priorities and a time-frame,'' Mr Hidding said.

``The State Liberals' Midland Highway policy has now adopted the priorities of that partnership agreement, however, we aspire to an acceleration of the rollout of projects.''

He said any bypassing of Perth or Campbell Town ``would likely be unnecessary''.