Hundreds of construction workers at Rio Tinto's Cape Lambert site have threatened to strike to get a new roster - which involves a pay cut - so they can spend more time with their families.
It is possibly the first time a WA workforce has threatened industrial action for a new deal that will reduce their annual salary rather than increase it.
The Australian Manufacturing Workers Union said the fly-in, fly-out workers would be at least $10,000 a year worse off under the proposed roster if their hourly rates remained unchanged.
Even if they achieved a generous 4 per cent increase in hourly rates, the workers would still get thousands of dollars less each year than they do now.
"The workers understand this could mean a drop in wages . . . and they are happy to give ground on this," AMWU secretary Steve McCartney said.
"Our workers know that family is the most important thing in life, which is why we are prioritising more family-friendly rosters in our enterprise bargaining agreement."
The workers, employed by subcontractors Laing O'Rourke and Monadelphous, want a week off for every three weeks on the job, instead of the current schedule of four weeks on and one week off.
Workers would spend five fewer weeks on the job each year, docking them five weeks pay.
Mr McCartney said the workers, including boilermakers and welders, were also driven by mental health concerns. "If you are north of the 26th parallel, you are seven times more likely to kill yourself than anywhere else in Australia, and those statistics are driving this whole thing."
Mr McCartney said he hoped the family-friendly roster would eventually be adopted across the sector.
Industrial relations experts say contractors are loath to reduce rosters because they would have to hire more people to do the same volume of work, increasing flight and accommodation costs.
A spokesman for Rio Tinto said it was up to the subcontractors to negotiate their own EBAs.