Unions warn WA wages campaign isn't over

The alliance of workers is pushing for a five per cent pay rise to combat rising inflation and cost of living pressures.

WA's government had initially offered a 2.75 per cent increase but last month upped the offer to three per cent, along with an additional $2500 sign-on bonus.

It was rejected by the alliance, which vowed to press ahead with industrial action.

Industrial Relations Minister Bill Johnston accepted an invitation to address Wednesday's rally on the steps of Parliament House in Perth, but was booed off the stage after telling workers the government's existing offer was fair.

"In the end, all industrial disputes come to an end," he said to a chorus of jeers.

"I want to make it clear that regardless of what happens ... the McGowan Labor government recognises and respects the work that you and your unions do."

The government's sweetened offer to 150,000 public sector workers was set to cost the state budget an extra $634 million over the next four years.

Premier Mark McGowan has insisted the new policy is his final offer, despite his government banking a $5.7 billion surplus in this year's stage budget.

United Workers Union WA secretary Carolyn Smith said the premier had "completely underestimated" workers.

"This is not the end. This is the beginning," she told the rally.

"We will not take a step back. We are here to look after the people of Western Australia and we deserve a fair pay rise."

WA Police Union president Mick Kelly said the government's offer fell well below Perth's 7.4 per cent annual inflation rate.

A recent union survey reported morale in the force was at an "all-time low".

"Our members say preventing crime doesn't pay ... (they) say that they're overworked and undervalued," Mr Kelly said.

"No copper has experienced 7.4 per cent growth in their base salary since this government came to power."

Unions WA secretary Owen Whittle described the offer as insulting, accusing the government of "leading a race to the bottom" on wages.