The West Australian MPs said the party was being realistic about its chances in Perth when it chose not to run candidates.

The foreign minister said even when the Liberals won 57 per cent of the statewide primary vote in WA, Perth voted for Labor.

"The seat of Perth is a Labor stronghold... we've got to be realistic," Ms Bishop told reporters in Perth on Monday.

"We're being pragmatic about it, we're having to fight by-elections across Australia as a result of Bill Shorten misleading Australians about the citizenship of his members."

Labor MP Tim Hammond resigned from his Perth seat for family reasons, while four other MPs resigned after a High Court ruling meant they were ineligible to be elected in 2016.

Senator Cormann said running candidates would be a waste of money.

"Whether you have a lot of money or lesser resources, it is never a good idea to waste money - you always ought to concentrate and prioritise your resources into those contests where you're most likely to be successful," Senator Cormann said in Perth.

State Director of the WA Liberal Party Sam Calabrese issued a statement on Saturday saying the party would focus on contesting the state by-election for the seat of Darling Range, but would field candidates for Perth and Fremantle in the "real" federal election next year.

WA Opposition Leader Mike Nahan said the seats were "real firm Labor" despite the party's margin in the seat of Perth being only 3.3 per cent.

"They've decided to reserve all their best people and funds for the real season rather than pre-season," Dr Nahan told reporters.

"And I think that's a logical decision and an appropriate one."

A date has yet to be set for the byelections.