Mr Barnett said he was disappointed independents Rob Oakeshott and Tony Windsor had decided to support a Julia Gillard-lead government despite Coalition leader Tony Abbott having clearly won the election.
He said the minority government formed by Labor, three independents and a Greens MP would be the "most left-wing government in our history on both social and economic factors".
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Ms Gillard had vowed to move ahead with the mineral resources rent tax (MRRT), Mr Barnett, Australia's only Liberal premier, said so it was all but certain it would be implemented.
"What we'll see is attempts to slow down the growth on the WA economy," he told reporters on Tuesday.
Mr Barnett said Labor may even increase the mining tax further under pressure from the Greens.
"The latest version was to tax only coal and iron ore. The Greens have made it clear they want to apply that tax right across the mining industry," he said.
"I expect to see the gold industry, alumina industry, the nickel industry all to be tax and that has immense implications for investment and long term employment in this state."
It also looks unlikely WA will sign up to the federal government's national health reform plan, with Mr Barnett declaring he will not hand over a third of the state's GST.
WA was the only state not to sign up to the health agreement in April, spearheaded by former prime minister Kevin Rudd.
"Basically WA will now have a separate health system from the rest of Australia," he said.
