The Liberal Party has won WA's Darling Range by-election and regained the seat vacated by disgraced former Labor MP Barry Urban, with voters punishing the McGowan Government in its first electoral test.

A big 9.1 per cent swing from Labor to the Liberals gave the party's candidate Alyssa Hayden 53.3 per cent of the two-party preferred vote after more than three-quarters of ballots were counted.

It means the former Upper House MP — who lost her seat at last year's state election — will be the next Member of Parliament in the traditionally-Liberal held electorate of Darling Range.

Mr Nahan, Ms Hayden and shadow minister Sean L'Estrange were all smiles after the win. ( Twitter: Sean L'Estrange )

Ms Hayden and her supporters celebrated the triumph at a party function in Byford on Saturday night.

Speaking in Byford on Sunday morning, she said her victory would start the process of "restoring some faith back into politicians".

"I am here simply because I went out on the street to listen to the community," she said.

"Being a good local member is about your community and delivering for the people and listening to their needs. It's as simple as that."

She also said she planned to move into the electorate.

Opposition Leader Mike Nahan said the vote spoke volumes about the Government's first year in office.

"It was a good victory, it was all about sending a message to [Premier] Mark McGowan that he has broken promises, increased fees and charges excessively, and cut frontline services," he said.

"It was always about sending a message to the Government that they have to change their ways."

Loading

Dr Nahan said the Liberals had a stronger candidate, and they were not afraid to make her the focus of their campaign.

"[Labor candidate] Tania Lawrence was never front and centre of the campaign, it was always Mark McGowan," he said.

"They hid her behind Mark McGowan the whole campaign.

"We did exactly the opposite. We had a really good candidate who was known in the electorate of course, and she was always front and centre of the campaign.

"I think Labor made a serious mistake in hiding her [Ms Lawrence]."

McGowan blames 'tough circumstances'

Mr McGowan and Ms Lawrence conceded defeat at a party function in Kelmscott, congratulating the Liberals and Ms Hayden.

Tania Lawrence and Mark McGowan conceded Labor had lost just after 8:00pm. ( ABC News: Jessica Strutt )

But the Premier rejected suggestions the loss was a vote of no confidence in his leadership and Government, and said the circumstances of the election impacted the results.

"I can't go and ask each of the 30,000 people who voted how and why they voted but clearly the circumstances were very difficult," he said.

"I think anyone who's reasonable would acknowledge that."

Mr McGowan said that there was usually a swing against governments in by-elections, which increases the impact of protest voting.

"A lot of people want to say, you know, a pox on all your houses, and I think that's probably what happened there."

Mr McGowan said he remained proud of Labor's campaign despite the result.

Loading

Labor's loss comes more than a month after Mr Urban resigned from the WA Parliament before he could be sacked for repeatedly lying about his past.

Ms Lawrence cast her vote at a polling booth in Mundaring, but it was not to be. ( ABC News: Eliza Laschon )

A powerful committee found the former Labor-turned-independent MP had lied to the public and Parliament for years about his police service and his education history.

Among the details rubbished by the committee was Mr Urban's claim he had served as a war crimes investigator in the Balkans.

He is now facing a criminal investigation by WA Police.

Sorry, this video has expired Barry Urban apologises to Parliament in November 2017 after resigning from Labor

Defeat caps troubled Labor campaign

Just weeks after Mr Urban's exit, Labor's first by-election candidate Colleen Yates quit the race when she admitted to misrepresenting her own education history.

Her campaign began to unravel when it emerged she had listed an "MBA" from the University of Western Australia on a now-deleted LinkedIn profile, even though she had not completed the qualification.

The events proved too much for the party's second candidate, Ms Lawrence, to recover from.

The resounding defeat still leaves the Government with 40 of the 59 seats in the Legislative Assembly after last year's election victory.

Turnout at the ballot box was 76.9 per cent, a figure ABC election analyst Antony Green described as "reasonable" for a WA state by-election.