Staff walked off the job at hospitals across Perth and regional WA on Friday in an escalation of their bid to secure improved pay and conditions.
The Australian Nursing Federation ignored a direction from the WA Industrial Relations Commission to call off the strike, which the McGowan government has warned will result in an "unprecedented" threat to patient safety.
About 3000 health workers rallied on the front steps of parliament, making themselves heard with air horns, cowbells and inflatable clappers.
The crowd then marched to nearby Dumas House where the office of Health Minister Amber-Jade Sanderson is located.
The building was in semi-lockdown on Friday with all but one entry closed.
Hundreds of category two and three elective surgeries have been postponed during the strike which is due to run until 9pm.
Thousands of outpatient appointments have been rescheduled or moved to telehealth, with oncology and dialysis patients among those affected.
Nurses and midwives have been offered a three per cent annual pay rise, a one-off $3000 bonus and the introduction of nurse-to-patient ratios.
The union last week encouraged members to accept the pay deal before reversing its position.
It now wants a five per cent increase which it says is necessary to combat rising cost-of-living pressures.
Speakers told the rally nurses were exhausted, frequently working double shifts and facing backlash from families over their loved ones being left waiting for care.
Graduate nurse Tom, who did not want his surname used, said he had been shocked by conditions at Sir Charles Gairdner Hospital.
He said he had seriously questioned quitting nursing, particularly with a local university offering a 12-month transition course to a better paid role as a paramedic.
"The ward which I'm on caters for the whole state. You discharge someone, then you've got someone who needs full nursing assistance," he told AAP.
"You go as fast as you physically can but you can't do everything.
"You have to hand over to the evening staff (things) which should have been done in the morning, critical stuff like wound dressing, medications.
"We don't want to strike but we haven't been given a choice."
Union state secretary Janet Reah labelled the premier and Industrial Relations Minister Bill Johnston "cowards" for not addressing the crowd.
She thanked nurses who had stayed back to work and promised the union would help cover any penalties incurred by strike participants.
Ms Sanderson earlier issued a statement urging the nurses union to return to the bargaining table.
"As a former union official, I support the right of workers to take industrial action, however, I cannot condone illegal activity nor activity which jeopardises patient safety," she said.
Only about 4800 union members took part in the vote to reject the latest pay offer, representing less than a quarter of the state's nursing and midwifery workforce.
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