Labor candidate Alannah MacTiernan has accused her Liberal opponent Darryl Moore of lying to voters over a $140 million pledge to upgrade the Tonkin Highway.

Earlier this month, Mr Moore and a Liberal colleague, Hasluck MP Ken Wyatt, each promised the funds to help the state government build three traffic overpasses along highway, a major freight route through Perth's northern suburbs.

Mr Wyatt made the promise in a press release on August 22. A week later, Mr Moore emailed potential voters to outline the plan, which would see traffic lights replaced with flyovers at three major intersections.

But the Opposition's official policy costings, released on Thursday by shadow treasurer Joe Hockey, contain no mention of the funding.

Ms MacTiernan described the pledge as "an obvious fraud that has been perpetrated on the voters of Perth".

She said she wanted to know why local candidates were making policy pledges "purporting to have the imprimatur of the Abbott team" when they do not appear in federal policy documents.

"What we need Mr Abbott to now explain is: are these roads in, and if they are in, why they are not in the policy document or the costings document? And if they are not in, how does he explain the conduct of Mr Wyatt and Mr Moore?"

A Liberal Party spokesperson said the Tonkin Highway works did not appear in the costings because the costings only deal with "new and accelerated" infrastructure plans, as well as detailing those that will definitely not be funded.

Tonkin Highway is not in either of those categories, the spokesperson said, adding that the project is "in the current forward estimates".

The federal Liberal campaign has promised the WA government $615 million to build the Swan Valley bypass, also known asthe Perth-Darwin Highway. But that does not include funds for the Tonkin Highway overpasses.

The Swan Valley bypass was among some $4 billion worth of road and rail policies that the Barnett government took to the state election in March.

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