Western Australia's health minister has criticised the state's ambulance operator amid a blowout in response times and ramping.

About 10 per cent of St John WA paramedics are on leave after becoming infected with COVID-19 or being identified as a close contact.

St John has activated a short-term plan allowing less-experienced ambulance officers to be dispatched in two-person crews to non-emergency jobs.

The officers are not recognised as paramedics by St John but have a degree in paramedicine. At least one must be registered with the medical watchdog.

Trained volunteers could also soon be deployed alongside paramedics.

Health Minister Amber-Jade Sanderson on Tuesday said there were paramedics isolating as close contacts who could return to work under the state government's critical worker policy.

"It would be my expectation that St John Ambulance is in lockstep with the public health system and that they're accessing that critical worker plan before they changed any mix of staffing within their ambulance service," Ms Sanderson told reporters.

"It's incredibly important that we maintain access to a quality ambulance service during this time."

Just 70 per cent of priority one emergency call-outs in March were responded to within 15 minutes, well below St John's 90 per cent target.

Job times have blown out due to staffing issues, additional infection control measures and a surge in ambulance ramping, where patients have to wait because emergency departments are not ready to receive them.

More than 30 ambulances were simultaneously ramped across the system on Friday night.

Health workers identified as close contacts can return to work if they are asymptomatic and return negative rapid antigen tests.

But they are not obliged to do so.

Employers are encouraged to only apply the policy in extreme circumstances and "after all attempts to deploy substitute staff have been exhausted", according to state government guidelines.

"Our ambulance personnel work in a mobile workplace and face-to-face with vulnerable Western Australians - often in private homes, aged care residences and hospital emergency departments," a St John spokeswoman told AAP.

"We are carefully working through operationalising our critical worker policy to ensure the safety of our people and patients."

Opposition health spokeswoman Libby Mettam said ambulance ramping had been at its worst in August last year when WA had negligible COVID-19 cases.

"The gridlock in our emergency departments is a result of the McGowan government consistently trying to cut corners on health," she said.

"The pressure has been building to the point where it is now having a dire flow-on effect on other services such as St John."