WA's Maritime Union of Australia is threatening industrial action against a third employer this week, with Esperance Port the latest to face the prospect of a strike.
The union applied to the Fair Work Commission on Friday for permission to ballot the port's 80 stevedores and maintenance workers about industrial action over a pay deal with Esperance Port Sea and Land.
The union has not yet revealed what type of industrial action would be considered by the port workers in a ballot.
A strike would force the port to a standstill and hold up millions of dollars of goods each hour.
A ban on overtime would not close the port.
The Australian Mines and Metals Association said the union was staging a "misguided industrial war against the resources sector".
It comes after the union's earlier threat to lead a week-long strike of tugboat operators employed by Teekay Shipping in Port Hedland, though it later agreed to delay the action that could have forced mine closures.
The union is also due to lead a two-day strike by 100 Tidewater cooks, cleaners and deckhands on projects across Australia from Tuesday.
But the mines and metals association is seeking to put a legal stop to the Tidewater strike by asking the FWC to withdraw its approval for the 48-hour stoppage, claiming the union's alleged failure to negotiate in good faith should render it unlawful.
The association's executive director Scott Barklamb said the body would provide evidence to the commission in Perth on Monday.
MUA assistant secretary Will Tracey refuted claims the union was not negotiating in good faith.
Union organiser Jeff Cassar said pay claims from the Esperance workers were not exorbitant. They want a 4 per cent annual wage increase and 2 per cent income protection claim to cover them for injuries outside of work.
Esperance Port Sea and Land could not be contacted on Friday.