The Australian Labor Party's monopoly on government ended Sunday when its branch in Western Australia conceded defeat to a coalition of conservative parties.

A week after a state election resulted in a hung parliament, the National Party threw its support behind Liberal leader Colin Barnett, allowing him to claim victory and become premier of the resource-rich state.

A power-sharing agreement not only tips Labor out of government but will likely signal the political demise of former state premier Allan Carpenter. He called the election a year early and is likely to pay dearly for a loss that ended Labor's dominance at federal and state level.

But the deal that made Barnett premier is also likely to make his job difficult.

"We are not prepared to go into a traditional coalition so we'll be accepting ministries based on being independent ministers, that reserve the right to exempt ourselves from Cabinet and vote against an issue on the floor of the Parliament if it's against the wishes of the people we represent," National Party leader Brendon Grylls said.

Carpenter, who lost the only election he contested, took over as premier from Geoff Gallop, who resigned citing depression in January 2006.

It was a big win for Barnett, who was drafted in to the party leadership little over a month ago when Troy Buswell was obliged to resign, after admitting to an unsavoury penchant for sniffing chairs vacated by female parliamentary colleagues.