Barely three months after campaigning for the election of the McGowan Government, WA's biggest public sector union has refused to sign its industrial agreement, citing a lack of trust.

The Community and Public Sector Union (CPSU) said its members would consider industrial action if the Government continued to oppose clauses relating to no forced redundancies and no privatisation of services in the agreement.

The union's state secretary Toni Walkington said her members were not prepared to take the Government at its word, because it had already broken its promise to give all public servants a pay rise of 1.5 per cent.

Instead, the Government offered capped pay increases of $1,000 per annum.

"Members are concerned that the McGowan Government has broken one promise, so they want the surety of these provisions in an industrial agreement so promises can't be broken into the future," he said.

"Our members are just not prepared to countenance a situation where our agreement doesn't include those provisions.

"It's now up to the Government to come back and address those concerns."

Ms Walkington said her members were willing to accept capped pay rises but warned industrial action was a possibility if the Government refused to write its pre-election promises in relation to job security into the agreement.

Community and Public Sector Union branch secretary Toni Walkington said the premier had already broken promises. ( ABC News: Andrew O'Connor )

"Things like work to rule, not doing overtime, only responding to emergencies, through to a whole range of different work bans, customer response bans, and ultimately I guess to withdrawal of labour," she said.

The CPSU was not the first union to flex its muscle with the new Labor government.

The WA Police Union commenced a work-to-rule campaign on Wednesday, accusing the Government of insulting its members with its new state wages policy.

The WAPU is also campaigning for the Government to fund stab proof ballistic vests for its members.

The Government was contacted for comment.