Roger Cook announced on Sunday he would add the defence industries ministry to his brief as leader of the Labor government after a frontbencher's resignation triggered an upcoming by-election.
In a snap press conference, Mr Cook said his priority was to diversify the economy in Australia's richest state, which accounts for about 17 per cent of the nation's GDP.
He said he wanted to lead the development of the largest naval maintenance and shipbuilding hub in the southern hemisphere, due to be developed in WA.
"We expect it (defence) is going to be our second biggest industry in Western Australia," Mr Cook told reporters.
"It is also one of the key portfolios which is outward projecting, which is about how we place ourselves and our reputation, working with our allies and our trading partners."
The state is undergoing a series of major naval expansion projects, including the development of a major commonwealth-owned defence precinct at Henderson, south of Perth.
A second precinct at nearby Garden Island will house a rotating force of UK and US nuclear-powered submarines before Australia begins operating its fleet.
Mr Cook also appointed himself to a new tourism, trade and investment portfolio to help promote WA internationally.
The ministerial reshuffle followed the retirement of then-corrective services minister Paul Papalia, leading to a looming by-election in his south Perth seat of Secret Harbour.
Mr Cook has conceded Labor will need to work hard to retain the previously safe seat, where it faces challenges from the Liberals and a resurgent One Nation.
Deputy Premier Rita Saffioti will also take on an expanded role, which includes responsibility for common-user infrastructure supporting the defence industry alongside her existing transport and major infrastructure ministries.
First-term MP Daniel Pastorelli, a former chief of staff to popular premier Mark McGowan, will assume responsibilities for the gilded portfolios of mines, petroleum and exploration, as well as water.
Mr Cook dismissed concerns about Mr Pastorelli's rapid ascension, saying he had sufficient experience to manage the important departments.
Mr Cook batted away suggestions Prime Minister Anthony Albanese would not be welcome in Secret Harbour, describing him as a "very popular politician" while downplaying the possibility the federal Labor leader might join the campaign.
"This is a by-election for a state seat - Albo is always welcome to come to Western Australia," he said.
David Mitchell will add state development to his ministries, Attorney-General Tony Buti will take on Mr Papalia's corrective services brief and Reece Whitby will add emergency services.
Georgia Tree, a former political staffer to federal Resources Minister Madeleine King, was announced as Labor's candidate for the by-election, due to be held on August 29.
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