'Poison coming across': One Nation faces test out west

Labor's stranglehold on the nation's richest state is about to be tested with the rising popularity of One Nation putting a safe seat in doubt.

Voters in the beachside seat of Secret Harbour, south of Perth, are heading to the polls on Saturday after long-serving MP and senior government minister Paul Papalia's mid-term retirement triggered a state by-election.

"The momentum appears to be with One Nation," political analyst John Phillimore told AAP.

But the devil could be in the detail, with a recent survey showing support for Labor holding steady at 33 per cent of the primary vote in Western Australia.

One Nation was up three per cent since June, running equal with the Liberals, with 21 per cent of the vote.

"One Nation support is not as high as it is in other parts of the country, so you'd have to be a little bit cautious about calling it early," Professor Phillimore, executive director of Curtin University's John Curtin Institute of Public Policy said.

"By the same token, in a by-election the government was always going to have a big swing against it."

WA Premier Roger Cook concedes it will be a battle to hold the outer suburban, mortgage-belt seat despite Labor's 11.5 per cent margin.

"At the moment, you see a surge on the east coast in popularity of One Nation," he told reporters on Friday.

Mr Cook was also quick to point out that very little rides on the result for his government, which won 46 lower house seats out of 59 in the 2025 state election - the party's third successive landslide victory.

One Nation's performance in tradie-haven Secret Harbour will be of interest across the nation, particularly Victoria, which has a state election in November.

It could have longer-term implications for senior federal Liberal MP Andrew Hastie, whose marginal seat of Canning takes in the Secret Harbour electorate and is being openly targeted by One Nation.

Mr Hastie, considered a future Liberal leader, has vowed to defend the seat which he retained last year on a reduced margin of 6.5 per cent.

One Nation is expected to soon announce a candidate for Canning in the next federal election.