Western Australia's Premier has poured cold water on the idea of expanding the state's Legislative Assembly, despite his party's support of legislation to do so in the Upper House.

A National Party bill to increase the size of the Lower House from 59 to 61 seats passed the Legislative Council on Thursday afternoon and would be enacted for the 2021 election if passed by the assembly by the end of the year.

But Premier Colin Barnett downplayed the prospect of that happening, saying it was not a priority and he did not expect it to come to a vote in the assembly this year.

If the bill is not passed by the assembly by the end of the year, it will lapse when Parliament is dissolved prior to the 2017 election.

Labor blasted the Liberals and Nationals for backing the bill, saying that paying an extra two MPs would be a waste of money and resources and that it showed the Government had the wrong priorities given cutbacks in spending in other areas.

But Mr Barnett said, despite the vote in favour of it by his own MPs, he was not expecting the idea to proceed in the short term.

"I see no circumstances in which it will be debated in this chamber," he told Parliament.

"It is not a priority for this Government at all."

But Nationals MP Martin Aldridge, who introduced the private member's bill, said it was needed in part to stop the decline in the number of regional Lower House electorates.

He also pointed to the geographical growth of some regional seats, with North West Central now around the size of New South Wales, and the rapid population growth in many metropolitan electorates as reasons for more MPs to argue the change was needed.

WA Nationals MP Martin Aldridge says electorates need to change to represent evolving demographics. ( ABC News: Jacob Kagi )

"In the last 10 years we have had 600,000 people come to Western Australia with no increase in the size of the Parliament," he said.

"Those new 600,000 people who call Western Australia home expect to have good access to their MPs."

Labor leader Mark McGowan said it was outrageous the Liberals and Nationals would seek to impose an "enormous expense" on the public by creating two more seats.

"They said there needs to be more regional politicians to represent people, but the National and Liberal parties have three regional MPs who have their offices in West Perth," he said.

"If you want to have more regional representation, how about you get those regional MPs to get those offices out of Perth and into the regions where they belong."

Electoral Affairs Minister Peter Collier said a bigger Legislative Assembly would prevent a further erosion of seats in the regions.

"What we are doing here is maintaining the integrity of the electoral system as it exists," he said.