The Australian Nursing Federation says it will only call off its planned state-wide strike on Friday if the WA government meets its demand of a five per cent pay rise.
The federation and the government have been locked in bitter negotiations over a pay and conditions deal for more than a month.
The latest offer from the government was met with in-principle agreement from the union last week.
But on Tuesday, 84 per cent of voting union members rejected that offer, and union state secretary, Janet Reah, called for a one-day strike to take place on Friday from 7am to 9pm across the public sector.
Ms Reah told ABC Radio Perth said there was only one outcome that would stop the strike.
"We will stop the strike if the government comes up with the five per cent across the board, plus the $3,000 cost-of-living payment, plus the ratios," she said.
"Otherwise it's a strike, nothing less will do now."
ED and ICU staff exempt
Some wards would be exempt from the action, including emergency departments, intensive care units and neonatal intensive care.
In the operating theatres, emergency cases would be "dealt with" according to Ms Reah, and only category one elective surgeries would go ahead, meaning a considerable number of category two and three surgeries would have to be cancelled and rescheduled.
"We're going to ask members to maintain minimum staffing levels in line with the night shift or low activity levels of staffing," she said.
"I think a sizeable portion of nurses that are on shift will strike.
"This strike on Friday is a snapshot of the future if we do nothing now."
The vote which led to the proposed strike was ordered to be deferred until next Monday by the Industrial Relations Commission (IRC).
Ms Reah went ahead with the ballot anyway and said the ANF is a "democratic union."
"For them [IRC] to say, we don't like the way you poll your members and even if you do, we're going to invalidate those results, it just beggars belief," she said.
Elective surgeries postponed
All category two and three elective surgery will be postponed tomorrow and Friday ahead of the planned industrial action.
Health Minister Amber-Jade Sanderson said the government was doing everything it could to minimise the impact.
"Some category one surgeries are likely to be impacted," the minister said.
"The reason it's Thursday and Friday is often people are brought in on Thursday for surgery on Friday."
Outpatient appointments, including oncology and dialysis, are likely to be rescheduled or moved to telehealth.
"We are very, very sorry to our community and to our patients, to the West Australian public patients, for this incredible, unprecedented and uncontrolled disruption to your healthcare by the ANF," Ms Sanderson said.
She lashed out at the union's handling of the wage negotiations.
"The ANF have demonstrated a complete inability to manage their membership or control industrial action to date," she said.
Strike will 'hurt' patients: Premier
Premier Mark McGowan said he was concerned about the impact the strike would have on the health system.
"There's no need for industrial action on Friday. Last week, the union agreed to the deal," he said.
"We put in place the arrangements they asked for. They repeatedly said 'if you give us this, we'll sign up'. We agreed to it. Then they didn't sign up.
"That happened on numerous occasions. How are you supposed to bargain like that?
"I just urge the union not to proceed the way they are. It can only hurt patients, hurt the community."