The union threatening to halt BHP’s iron ore exports says its members earn more on government infrastructure projects in the city.

BHP could lose $100 million in revenue a day if two unions vote for work stoppages, unless it can find about 450 replacement staff in Western Australia.

Hundreds of BHP Port Hedland staff are a step closer to walking off the job at Australia's biggest iron ore hub.

The action would be the first strike in the Pilbara in decades, and the alarms were sounding for those in the mining industry around pending industrial action as far back as March.

The union threatening to halt BHP’s iron ore exports says its members earn more on government infrastructure projects in the city.