The Barnett Government started the first full week of the 2017 election campaign levelling claims of "fake funding" at Labor's Metronet project and finished it in a rear-guard attempt to defend its economic record in the face of a damning assessment from its own Treasury.

It did not take long for the Trump-like accusations of fake facts to surface in the campaign, with Treasurer Mike Nahan unrestrained in his attempt to discredit and derail Labor's signature election policy — the Metronet public rail expansion.

"Fake funding to pay for Metronet" screamed the headline of the Treasurer's media release, as he catalogued Treasury estimates suggesting Labor's costings were wrong and indicating the project could cost up to nearly $10 billion.

Mark McGowan faced attacks this week over WA Labor's Metronet costings. ( ABC News: Andrew O'Connor )

"It is a fraud on the public of Western Australia, particularly the people of Ellenbrook, and it simply will not happen unless they increase debt substantially," he told reporters.

Just hours before, WA Labor's leadership team of Mark McGowan, Transport spokeswoman Rita Saffioti and shadow treasurer Ben Wyatt had unveiled the costings for stage one of the Metronet project, putting the price tag at $2.5 billion.

For that figure, Labor pledged to build four new rail lines, a number of new stations and remove four level crossings, as well as boost rail efficiency through improved signalling.

Labor's estimated cost of $2.5 billion contrasted sharply with the price range put forward by the Treasurer of $6.76 billion to $9.75 billion, based he said on Treasury analysis.

But the Government's strident attack was quickly blunted when it became clear it was targeting the original 2013 version of Metronet, many elements of which had been stripped out of the version Labor was taking to next month's election.

Labor brands attack 'disgraceful, embarrassing'

Dr Nahan's figures claimed a $1.7 billion cost for removal of 30 level crossings. Labor had promised only four at a cost of $257 million.

The figures included a heavy rail tunnel from the city to Morley and a rail line to Ellenbrook with a cost of $3.88 billion. Labor had promised a line from Ellenbrook to Morley, joining the existing Midland line to the city at a cost of $863 million.

Dr Nahan's cost estimate also included a rail line connecting the Joondalup line to the proposed Ellenbrook line, cutting across the northern suburbs at a cost of $884 million. Labor's 2017 Metronet plan contains no such proposal.

Labor was scathing in its response, describing the Government's attacks on the Metronet plan as "disgraceful" and "embarrassing".

It was equally hostile to the Treasurer's claims that the funding sources for the project were "fake", with Dr Nahan casting doubt on how Labor would raise $667 million from land sales, $267 million from so-called value capture, and $415 million in Commonwealth funds from a cancelled Perth Freight Link.

Without that cash, Dr Nahan argued, Labor would add $2.4 billion to the state debt.

Labor defended its forecasts, pointed to the fact its costings had been independently verified, and explained that its projections had been supported by the private sector.

Liberals' focus turns from Metronet to state debt

As the Treasurer found himself fending off questions about making false claims about Metronet, Labor made daily announcements, detailing the cost and timelines for the proposed new lines and offering data to underpin its arguments.

Mr McGowan visited Ellenbrook on Friday to promote Metronet and a rail line from Midland. ( ABC News: Andrew O'Connor )

Within 72 hours, the Government's Metronet assault had all but run out of steam.

But by Thursday, the Barnett Government had a fresh set of figures to worry about in the form of the Treasury's pre-election Financial Projections Statement.

Released by Under Treasurer Michael Barnes almost four weeks out from election day, it was anything but a beautiful set of numbers.

Gone was the projected surplus at the end of the decade, which had been trumpeted by the Government in its Mid Year Review in December, instead leaving the state budget in deficit for the foreseeable future.

The forecast for state debt continued to climb, with the figure projected to balloon beyond $41 billion by the 2020.

Labor seized on the numbers as the final evidence of the Barnett Government's failure to competently manage the state's finances.

Treasury 'unnecessarily pessimistic': Barnett

But on Friday, as Premier Colin Barnett outlined a spending initiative to widen sections of the Mitchell Freeway, he seemed untroubled by the debt, deficit or Treasury's bleak assessment of the state's economic outlook.

The WA Premier maintained his Government had the best plan to reduce debt. ( ABC News: Andrew O'Connor )

Mr Barnett shrugged off a question about whether the figures shredded the last remnants of his Government's reputation as economic managers, saying debt would be reduced by selling Western Power, and the deficit returned to surplus in the next term.

He did not, however, suggest the Treasury forecasts were fake. Just too bleak.

"I don't query Treasury. [But] I think their prognosis for the future of the West Australian economy is unnecessarily pessimistic. Our economy is far stronger than that," he said.

WA voters have four weeks to make up their own mind who they believe.