Until Friday, health officials had been guarded about whether people should wear masks.

But with Victoria's numbers soaring to an Australian record daily increase of 288 cases, people in lockdown areas are now being encouraged to use them.

The recommendation is for people in metropolitan Melbourne and the Mitchell Shire when social distancing is not possible.

"Through an abundance of caution, through the fact it is relatively easy, we should request of all Victorians when they are out of their home, and this relates principally to metropolitan Melbourne and Mitchell Shire ... it is our request of you (to wear a mask)," Premier Dan Andrews said.

"It's not compulsory, we are simply asking that if you can wear a mask where you can't distance, that is exactly what we would like you to do.

"That's a relatively small contribution but one that could make a really big difference."

The state government will order two million reusable masks and one million single-use masks to help cope with the added demand.

State chief health officer Brett Sutton said studies made public in the last couple of weeks had shown that masks can reduce transmission of the virus by about 60 per cent.

Prime Minister Scott Morrison added that national cabinet, as of their Friday meeting, had adopted the revised health advice about masks.

"In those circumstances that the (national) chief medical officer has outlined, it is not a broad application across the country but in designated areas that have been affected in the way that they have in those parts of Victoria," Mr Morrison said.

"That is the advice and that was adopted by national cabinet today."

Prof Sutton has told residents to be crafty and make their own face masks instead of buying them.

"I think people will need to make their own masks," he said on Thursday afternoon.

"I don't think there should be a rush on buying single-use masks, for example, and some masks that are able to be purchased ... aren't necessarily good for rewashing and reuse over days and days."

The state's health department is working on providing information on how to make a basic mask that can be reused and rewashed for several weeks.

Medical experts are divided on the use of masks, with some saying they only provide a marginal benefit.

But Dr Abrar Chughtai, the director of the Master of Infectious Diseases Intelligence program at UNSW, wants their use to be much more widespread.

"I think masks use should be mandatory in Melbourne, particularly in high transmission areas and suburbs, and public and crowded places," he said.

Australian Associated Press