Here's what you need to know about the Victorian government's move.
Who needs to wear a mask?
Residents in metropolitan Melbourne and the Mitchell Shire will be required to wear a face mask or covering when they leave their homes for any of the four legal reasons.
Mr Andrews said "common sense" will guide the new rules, and despite being mandatory, he advised the measure be done "whenever practical".
"If you are out of your home for one of the four reasons, then you need to be wearing a mask and I stress ... it need not be a hospital-grade mask," he said.
"Any face covering is better than no face-covering."
Under Victoria's lockdown restrictions, the four reasons that residents of a coronavirus hotspot can leave their homes include shopping for food and essential goods or services, care or compassionate reasons, exercise, or work or study, if you can't do it from home.
Mr Andrews said the move was the "powerful next step" in managing the spread of coronavirus in the state.
"It doesn't come at an enormous cost to the Victorian economy," he said.
"It still allows us to go about our business — particularly those who can't work at home and for going shopping for the basics when you need them."
Victoria has seen a spike in coronavirus cases over the past month, which has resulted in a rise of the state's death toll to 38, taking the national toll to 122.
The mandate was introduced in response to the virus spreading in workplaces.
"About 80 per cent of our new cases since mid-May are being driven by transmission in workplaces, including private sector aged care," Mr Andrews said.
The premier said that residents in Victoria and potentially in other parts of the country will be wearing masks "for a very long time".
"There's no vaccine to this wildly infectious virus and it's a simple thing, but it's about changing habits, it's about becoming a simple part of your routine.
"Most of us wouldn't leave home without our keys, we wouldn't leave our home without our mobile phone. You won't be able to leave home without your mask."
On Sunday, Federal Health Minister Greg Hunt said Victoria's mask move was "necessary".
"We are sorry that it has reached this point for all those who were affected. But this is about saving lives and protecting lives.
"The advice from the AHPPC [Australian Health Protection Principal Committee] has been consistent, and what has changed is the level of community transmission in Victoria."
What happens if you don't wear a mask?
As the move is mandatory, police can impose a fine of $200 for anyone not wearing a mask or covering, but there are exemptions.
The premier offered scenarios to when people can remove their masks but encouraged residents to exercise "common sense".
"If you're out running, part of your daily exercise, then it may not be practical to wear the mask while you are running, but you should bring the mask with you and you should wear it before and after you have gone for your run.
"There'll be some other environments, where you're going into a bank, for instance, then you would need to take your mask off. If you worked at a call centre, then it may be very challenging for you to perform your duties wearing a mask."
What if you can't get a mask?
The premier confirmed that up to three million face masks are on order for the state and the first batch of 300,000 is due to arrive this week.
He said updates will be provided this week as to how those will be distributed.
Mr Hunt said an additional one million masks would be provided for disability workers across Victoria and an extra one million would be provided for regional aged care facilities.
Victorian Health Minister Jenny Mikakos said information to create your own cloth masks is available on Department of Health and Human Services website.
But she advised people not to be "stressed or anxious" if they cannot get their hands on a top-tier surgical mask.
"You can make one yourself. You can use a scarf, you can use a bandanna, something is better than nothing."
Who is exempt?
Children below the age of 12 will not be required to wear face masks or coverings, but it will be mandatory for high school students.
Teachers will not be required to wear masks in the classroom, and they will not be required in childcare centres.
Victoria's Chief Medical Officer Professor Brett Sutton said there would be exemptions for people with legitimate excuses, such as medical reasons.
He said exemptions will also be given for people engaged in an activity where it is "entirely impractical" to wear a mask.
"But it does make a difference. And having universal mask-wearing is the key here."
Residents in metropolitan Melbourne are subject to stay-at-home orders and can only leave home for essential work, study, exercise or care responsibilities. People are also advised to wear masks in public.
People in Australia must stay at least 1.5 metres away from others. Check your state’s restrictions on gathering limits.
If you are experiencing cold or flu symptoms, stay home and arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.
News and information is available in 63 languages at sbs.com.au/coronavirus