Australian Workers' Union members voted against Alcoa's proposed enterprise bargaining agreement (EBA) presented this month.
Around 1600 workers are on strike after it emerged that Alcoa had applied to the Fair Work Commission to have current employment terms and conditions terminated.
The AWU says this would leave only basic terms and working conditions.
Negotiations over a new EBA have been ongoing for 18 months.
Alcoa said the proposed EBA offered income growth on top of already competitive pay, 14% superannuation, and income protection for up to two years in the event of extended illness.
Following Friday's vote, Alcoa said it would welcome an alternative proposal from the AWU that would support its aim to establish a competitive agreement.
"We understand and are concerned by the impact the continued and unnecessary industrial action is having on our employees," Alcoa said.
"We invite them back to work so they can restore their livelihoods while we work to bring this matter to a resolution."
Alcoa said the sites had contingency plans to ensure continued operations during the strike.
The company estimates the industrial action impacted alumina production at WA refineries by around 15,000 tonnes in August.
The company produces around 9 million tonnes a year.