Western Australia's Opposition Leader Mike Nahan says the Government should break a key election promise to help plug a budget shortfall.

Earlier this week, the Liberals decided to block the Government's proposed gold royalty hike, leaving it dead in the water without the Nationals or crossbench support.

It leaves the McGowan Government scrambling to fill a $400 million hole in the state's budget.

It has warned it may have to hit households with more fees and charges or introduce a new bank tax, but Dr Nahan said it should cut spending instead.

He suggested the Government scrap parts of its flagship transport plan, Metronet.

"They have an expenditure problem," Dr Nahan said.

"The cost of Metronet from the pre-election has expanded by a billion dollars [and] they've left out a large section of Metronet that's yet to come."

The Opposition Leader then said it would not be unusual for a government to pull back on its election promises after true costings were realised.

Transport minister Rita Saffioti said they will deliver Metronet. ( ABC News: Eliza Laschon )

"In 2013, we promised MAX Light Rail and the more we looked at it, the more it blew out in cost," Dr Nahan said.

"We broke a promise and we were pilloried ... [but] if you do not have the money and the costs are blowing out and there's no business plan like there is none for Ellenbrook, you have to cut back not raise taxes."

But Transport Minister Rita Saffioti has slammed the suggestion the Government should scrap part of its transport plan.

"They're all over the place," Ms Saffioti said.

"They've blocked the gold royalty because they've said it's a broken election promise and now they want us to break more election promises.

"We'll be delivering our election promises to build the Ellenbrook rail line and to build Metronet."

Metronet includes plans for rail lines to the Perth Airport and Ellenbrook in the city's north-east.