Masks while walking around at events of more than 30,000 people — including the Boxing Day Test — are also mandated, but not while seated.
People are also recommended to work from home when possible, and hospitality venues are encouraged to switch to seated service only, but neither of those changes have been mandated.
The changes come a day after Prime Minister Scott Morrison said masks were “highly recommended”, and on the same day NSW recorded more than 5700 new cases of coronavirus, while cases in Victoria crept over 2000.
Victorian Chief Health Officer Brett Sutton said Victoria was in a “fantastic” position thanks to high vaccination rates.
“Masks are a very small individual cost for us and they have a terrific effect on transmission,” Prof Sutton said.
“It’s really critical. To wear them outdoors moving around at big events is a big deal.”
He said data coming out of the United Kingdom showed while the Omicron variant may be as much as five times less severe than previous variants, it spread more than 15 times more quickly.
Prof Sutton said that meant the sheer number of case numbers could still overwhelm the health system.
Acting Victorian Premier James Merlino said they were “common-sense, sensible changes in response to the new variant” and said the government was not considering a lockdown.
Mr Merlino said Victoria was “in one of the best positions of any place in the world” to combat the Omicron variant.
He also said the Victorian Government would invest $31 million into preparing for a rollout of third COVID-19 vaccine doses, and again called on the Australian Technical Advisory Group on Immunisation to shorten the interval between second and third doses.
“The interval needs to be shortened and it needs to be shortened immediately. It’s the clear public health advice we’re getting,” Mr Merlino said.
“These changes will allow businesses to stay open and it is a cheap and effective way to maintain the health of the community.”
Victorian Health Minister Martin Foley said the orders would run at least until January 12, and said the scale-up of the booster program would not delay children’s vaccinations.
Earlier this week at National Cabinet, Prime Minister Scott Morrison said states would review the requirement for PCR tests when travelling interstate.
"One in four people waiting in those queues are not symptomatic. They're not a close contact. They're not even a casual contact. They just want to travel to another state," Mr Morrison said.
"This is putting unnecessary pressure on the testing system, and it is redirecting resources away from where there is better use of them.
Mr Morrison recommended people wear masks indoors but stopped short of pushing for a national mandate.
National Cabinet also discussed reopening vaccination sites across the country in preparation for an expected two million third doses per day by January 10.
NSW introduced a mask mandate later on Thursday, as well as a limit of two people per square metres until January 27.
