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Labor wants its ambitious Metronet costed by Treasury but the Government has refused its request to allow Under-Treasurer Tim Marney to independently analyse the project.

The public transport blueprint remained in the spotlight yesterday as Opposition Leader Mark McGowan promised his third new rail line in as many days.

Mr McGowan's announcement of a $616 million, 12km north circle line including new stations at Balcatta, Hamersley, Mirrabooka and Malaga comes after a promised $731 million airport spur and $863 million Ellenbrook line.

With the financial implications of Metronet mounting, Premier Colin Barnett and Treasurer Troy Buswell have said Labor must explain how the numbers will stack up.

But shadow treasurer Ben Wyatt released to _The West Australian _a November letter from Mr Marney confirming the pair had discussed costing Opposition policies early.

Under current conventions, Treasury can cost Opposition policies only after the release of the pre-election financial projections statement. The statement updates the State's financial position from December's mid-year review and must be released by Mr Marney 10 days after the issuing of the election writs, scheduled for next Wednesday.

Mr Marney wrote: "I have been advised that if you would like to propose amendments to the conventions, that you discuss the conventions with the Premier." Mr Wyatt said WA's shift to fixed terms meant the Opposition was denied access to its chief information source, Parliament, for more than four months leading up to an election.

Liberal candidates have released statements quoting "independent costings of Treasury" putting a $6.4 billion price tag on Metronet.

Labor has costed its eight-year "priority" Metronet program at $3.8 billion. Mr Wyatt said it was outrageous the Government had used the public service to cost Metronet before Labor confirmed its route or timeline, released the costing in a "highly political way" and stymied attempt to clarify it.

"The Opposition will have the assumptions underlying its costings assessed by a third party and ask the Treasury to assess the impact of the estimated cost of commitments against the State's financial aggregates," he said.

Mr Barnett declined to comment.