Three Victorian hospitals are warning people not to visit emergency departments unless "absolutely necessary" due to extreme pressure from rising COVID-19 admissions and demand for testing.

Health authorities say the state will see a "rapid" increase in coronavirus hospitalisations in the coming days.

The number of people in hospital with the virus has risen by 123 in the past week, from 368 on December 27 to 491 on Monday.

Victoria's seven-day hospital average jumped by 20 cases within a day, from 424 in Sunday's figures to 444.

Western Health, which manages Sunshine, Footscray and Williamstown hospitals in Melbourne's west, says all three are "currently under extreme pressure" as they warned people not to attend for COVID tests.

"Please DO NOT attend the ED unless absolutely necessary," Western Health tweeted on Monday.

"Those with mild COVID symptoms seeking PCR or rapid tests should NOT ATTEND. Thank you for your cooperation."

Health Minister Martin Foley said all of the state's emergency departments were facing "unprecedented demand" which was growing every day.

"We are seeing business as usual - for want of a better phrase - demand continue to grow as people get out more and as people come forward for all the normal things that a health service needs to deliver," he told reporters on Monday.

He described Western Health's response as "sensible" and suggested other health networks were adopting the same approach

Victoria's COVID-19 commander Jeroen Weimar said he expected coronavirus hospitalisations would "increase quite rapidly in the days ahead".

"If we look at the experience in New South Wales, who appear to be about a week or so ahead of us in this pandemic, they're now starting to report quite significant numbers," he said.

The number of people critically ill with COVID-19 in hospital has remained stable, with 56 actively infected in ICU as of Monday and 24 requiring ventilators.

However, Mr Foley warned ICU numbers could also rise.

"If you work on the basis that we're a week or two behind where New South Wales are now, we can't rule out that we will continue to see an increase in COVID being managed at home in the community, an increase in hospitalisation, an increase in ICU and an increase in ventilators," he said.

"These are not good prospects to be looking at over the course of the coming month in January."

He urged people to get their COVID-19 vaccine booster dose as soon as they are eligible, and continue to monitor public health and social measures.