Alyssa Hayden, a two-term Barnett government upper house MP, beat Labor candidate Tania Lawrence, a former Woodside manager.

The by-election was triggered by the resignation of Barry Urban after colossal lies about his background were uncovered, including wearing a police medal for investigating war crimes in Bosnia despite never having served there.

So it was gobsmacking when the government found itself embroiled in another credibility crisis with its first candidate, Colleen Yates, who apparently didn't fib to the party but committed the lesser crime of exaggerating about her tertiary qualifications in an online profile.

"It was a very tough by-election and very tough circumstances as we are all aware," Premier Mark McGowan told party members on Saturday.

He said the "circumstances" were beyond Labor's control but he had previously admitted the party's vetting process had to be improved, insisting Ms Lawrence had been subject to a higher level of scrutiny when unveiling her as the new candidate three weeks ago.

He blamed the loss on the controversies, saying they had clearly hit Labor in the polling booths although it was always hard to win a by-election in government.

"The result is one we will review, clearly, in coming days and weeks, and take heed of what the people of Darling Range have said to us."

During last-minute campaigning, Ms Hayden seized on the controversies plus a fresh one that erupted on Thursday when Water Minister Dave Kelly allegedly made a headbutt gesture at Nationals leader Mia Davies during a heated debate in parliament.

Ms Hayden described the gesture, which Mr Kelly vehemently denies making and was not caught on camera, as "disgraceful".

"This is a chance for the people of Darling Range to say we will not tolerate bad behaviour, we will not tolerate lies and broken promises," she said.

"Labor are not fit to govern in my opinion."

She said voters in the traditionally Liberal-held seat had indicated they were deeply disappointed by the Urban scandal.

The Liberals also campaigned against big hikes in household fees and charges, which kick in on July 1, but Labor defended that argument by saying it was cleaning up the financial mess left by the previous government.

Both parties had said they expected the poll would be a tight race, but Opposition Leader Mike Nahan had leaned on the side of pessimism, saying toppling Mr Urban's 5.8 per cent swing was "a big ask".

Ms Lawrence, who made it to polling day with her reputation intact, said she would have another go at politics at future elections.

Labor still has a massive upper house majority of 40 out of 59 seats.

Australian Associated Press