Colin Barnett's eight-and-a-half-year reign as WA Premier is over, after his Liberal-National Government was swept out of office by a huge swing towards Mark McGowan's Labor Party.

Key points: Labor expected to win as many as 41 seats, having already secured more than the 30 required to form government

Labor expected to win as many as 41 seats, having already secured more than the 30 required to form government Mark McGowan will be sworn in as WA's 30th premier within days

Mark McGowan will be sworn in as WA's 30th premier within days Three Liberal ministers, including John Day, have lost their seats

Mr McGowan will be sworn in as the state's 30th premier within days, after Labor won a clear majority in parliament and the Liberals suffered disastrous losses above even its most dire fears going into the election.

The ABC called the result less than 90 minutes after polls closed, with early counting showing dramatic swings to Labor.

ABC election analyst Antony Green said he expected Labor to win as many as 41 seats, having needed just 30 to defeat the Barnett Government.

Three Liberal ministers, including Health Minister John Day, have lost their seats while Corrective Services Minister and possible leadership aspirant Joe Francis is also in severe trouble.

The National Party's vote only fell slightly, but leader Brendon Grylls's seat is still uncertain.

In celebratory remarks made to a raucous gathering of Labor supporters in Rockingham, Mr McGowan thanked Mr Barnett for his "many years of public service to West Australians".

Mr McGowan pledged to get to work immediately, as he reaffirmed his commitment not to privatise Western Power.

"Today West Australians voted for hope and opportunity over desperation and division," the Premier-elect said.

"Today we showed we are a state of decency and intelligence, not a state of stupidity and ignorance.

"The core Australian values of equality, fairness, merit and opportunity shone through in this election campaign."

Labor's victory 'emphatic': Barnett

Sorry, this video has expired Watch in full: Colin Barnett concedes defeat ( ABC News )

Making his concession speech less than three hours after the polls closed, Mr Barnett congratulated Labor and said his opposition had won an "emphatic and decisive victory".

"We ran, in my view, a great campaign, but time was probably against us," Mr Barnett said.

"Politics is a brutal, harsh business.

"I can assure you that I have given this our best shot in every sense."

Mr Barnett said he would leave the post-election analysis to others and did not comment on the party's controversial preference deal with One Nation, which he had previously admitted to feeling "uncomfortable" with.

With more than half of the vote counted, the Liberals have lost almost 16 per cent of their primary vote — putting them on course to suffer the biggest-ever swing against a sitting government in WA.

It is also shaping as a disappointing night for One Nation, with Pauline Hanson's party receiving just 4.7 per cent of the primary vote so far — less than half of what it had been projected to receive in pre-election polling just weeks ago.

But One Nation is polling slightly better in the Upper House and is still in the mix to win seats in that chamber.

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Among the Government seats the ABC is projecting Labor to win, Mr Day, Local Government Minister Paul Miles and Environment Minister Albert Jacob have all lost their spots in Parliament.

In a huge shock, Mental Health Minister Andrea Mitchell is also in severe danger of losing her previously very safe seat of Kingsley.

Incredibly, the previously very safe seat of Murray-Wellington also appears to have fallen into Labor's hands while a massive swing has left Mr Francis' seat of Jandakot in doubt.

The Liberals are also projected to lose Southern River, Mount Lawley, Perth, Forrestfield, Morley, Swan Hills, Collie-Preston, Balcatta, Bicton, Belmont, West Swan and Darling Range.

There are also big swings in Geraldton, Joondalup, bringing all of these Liberal-held seats into play.

Labor is also polling strongly in Pilbara, with Mr Grylls likely to face a long wait to see if he can hold onto his spot in Parliament.

There was little good news for the Liberals, although Treasurer Mike Nahan has held onto his seat of Riverton and Peter Katsambanis beat independent Rob Johnson in Hillarys.

Mr Francis declared he would seek the Liberal leadership if he retains his seat. He finished the night about 50 votes behind Labor's Yaz Mubarakai.

Federal Justice Minister Michael Keenan admitted the result was a "savage message" for the Liberal Party but denied it was any reflection on the Turnbull Government.

"Penalty rates and federal issues have been no issue in this campaign," Mr Keenan said.

Pre-election polls indicated Mr Barnett's Government was likely to be defeated by Mr McGowan's Labor Party.