The emergency procedure was implemented about 2am on Sunday and lasted 90 minutes, during which metropolitan Melbourne residents were urged not to call for help unless in a serious condition.

Staff were forced to operate at limited capacity, while restricted access to triple zero meant some calls were potentially unanswered.

The crisis protocol saw managers return to duty to manage hospital transfers, rapid offloading of patients at hospital and lower acuity patients referred to other services.

Additional staff were recalled and non-emergency ambulances responded to some cases.

It comes amid fresh concerns about COVID-19 outbreaks and the BA.4 and BA.5 Omicron sub-variants, along with staffing shortages across the healthcare sector.

At least seven code reds have been declared this year.

Lindsay Mackay, Ambulance Victoria's executive director of operational communications said COVID-19 had significant impact on the service.

"We've had approximately 170 paramedics that have been furloughed every day this week due to COVID and we're just not immune to it, like anybody else," Ms Mackay said.

"So we are experiencing that as an ongoing challenge and obviously with the flu season it will continue to be a challenge."

Opposition Leader Matthew Guy says the state government is to blame for the situation.

"What we had last night again shows this government's management of our health service has been abysmal," he said on Sunday.

"They've had two years in the world's longest lockdowns to prepare us. They've clearly done no work or prepared nothing and the end result ... is we now have our seventh code red this year, and that is outrageous."

Ambulance union secretary Danny Hill has previously said the system is struggling.

"The system is busier than it has ever been and that's not because of an increase in strokes, heart attacks or car accidents, it's because of an increase in reliance on triple zero," Mr Hill said.

"It means (paramedics) won't be finishing their shifts on time or getting a meal break and will spend a large part of their shift ramped at the hospital because there's so many patients they're bringing in."

Thousands of Victorians could join a lawsuit against the Emergency Services Telecommunications Authority following a series of fatal system errors that have led to at least 15 deaths and multiple injuries, some involving children, since 2014.

Meanwhile the opposition continues to criticise what it says are mixed messages around COVID-19 restrictions after conflicting advice on the reintroduction of mask mandates and work-from-home orders.

A day after new Health Minister Mary-Anne Thomas refused to rule out mask-wearing or work-from-home orders, Industry Support and Recovery Minister Ben Carroll told reporters on Friday the government was not considering mandates and lockdowns.

"The Andrews Labor government is in a state of chaos over COVID rules," Mr Guy said.

"This chaos is sowing confusion and fear in the Victorian community, who are fearful of the damage that a return to indoor mask mandates, work-from-home recommendations and potentially worse restrictions will do to Victorians' lives, the small business sector and the Melbourne CBD."

Australian Associated Press