Another Melbourne school has been closed after a teacher who recently returned from the United States was diagnosed with coronavirus, as the Victorian Government activated the State Control Centre to respond to the epidemic.

Key points: Three of the state's four new cases had recently returned from the United States on separate flights

Three of the state's four new cases had recently returned from the United States on separate flights Yeshivah-Beth Rivkah Colleges has closed for the day due to a teacher testing positive for COVID-19

Yeshivah-Beth Rivkah Colleges has closed for the day due to a teacher testing positive for COVID-19 The chief health officer said Victoria would likely have to implement "really disruptive" measures to stop the spread within weeks

The male teacher from Yeshivah-Beth Rivkah Colleges, aged in his 50s, is one of four new confirmed COVID-19 cases in Victoria, which has brought the state's total number to 22.

He became unwell on March 7, one day after arriving back in Melbourne from Los Angeles and worked at the school's St Kilda East campus for half a day while infectious on Monday.

The orthodox Jewish school and its associated early learning centre, creche and Yeshivah Centre were closed on Tuesday as a precautionary measure.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 33 seconds 33 s The Yeshivah-Beth Rivkah school community has been asked to self-isolate while authorities notify close contacts of the infected teacher.

Four families from the school community who happened to be on the same flight have also entered self-isolation, the principal said.

It follows the suspension of classes at Carey Baptist Grammar School after a teacher there tested positive to the virus in the first recorded case of person-to-person transmission in Victoria.

Carey's principal Jonathan Walter said in an email to staff on Tuesday afternoon that the school learnt at 1:30pm that a second staff member who shared an office with the infected teacher had since tested positive for COVID-19.

He said the school, which would remain closed this week, was contacting all individuals identified as close or casual contacts with the staff so they could take steps to manage their potential exposure to the virus.

The second Carey staff member took the state's official COVID-19 case number to 22, after authorities announced three new cases on Tuesday morning in people who had recently returned from the US.

Severe disruptions could start in weeks

Victoria's chief health officer Brett Sutton said he expected it would be "weeks, rather than months" before the number of person-to-person cases in Victoria prompted authorities to bring in "really disruptive public health measures".

"And it might be within the next four weeks. But I'm not going to pre-empt when it will be, because we are going to make an assessment every single day," Dr Sutton said.

Premier Daniel Andrews on Monday warned "extreme measures" would likely be needed when the outbreak rose to pandemic phase.

Seven Victorian hospitals are testing for coronavirus, including the Royal Melbourne Hospital in Parkville. ( ABC News: Ron Ekkel )

He said it was only a matter of time before schools closed across the state and large sporting and cultural events were cancelled.

"It remains the case that we can with certainty foreshadow extreme and widespread disruption," Mr Andrews said today.

"Big challenges for our economy, for schools, for workplaces, for major events, but it remains the case that that is not now. The time for that will be at some time in the medium-term.

"It's not a matter of panic or wanting to scare people, it's simply a matter of being as honest and upfront as we can be."

He said planning was in place to ensure "potentially many tens of thousands" of public servants would be able to work from home if needed.

Mr Andrews said the specific details of disruptions would need to be worked out as the situation developed.

"I understand that people would like answers to all of these questions," he said.

"It is frustrating that we can't give you absolute dates and a really clear picture of what might occur, but it'll be based on advice and it'll have to be guided by the circumstances that we face."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 45 seconds 45 s AFL chief executive Gillon McLachlan yesterday confirmed the league would play games "with no crowds" if required.

Emergency Management Commissioner Andrew Crisp said he and his team decided the "time was right to stand up" the State Control Centre.

Mr Crisp explained that under the state's emergency management protocols there were three tiers of emergency response and the virus was now considered a Tier 2 emergency.

He said this summer's bushfires were considered a Tier 3 emergency — the highest level — for about 90 days and were only last week dropped to Tier 1.

He said the response to the virus would be escalated to Tier 3 when there were "much greater consequences to much broader parts of our community".

Dr Sutton said it was possible a state of emergency could be declared under the Public Health and Wellbeing Act, which would give him the power to quarantine groups of people or place areas under social distancing measures.

The Premier also has the ability to declare a state of disaster — like the one that was declared during the bushfires — which Mr Andrews said could give "a broader suite of powers" to control the delivery of services, including public transport.

People can be detained or fined for failing to comply with orders under both a state of disaster and a state of emergency.

New case attended seafood festival, rugby match

Victoria's 19th confirmed case is a man in his 50s who returned to Melbourne from Seattle on March 7 at 7:00 am on Virgin Australia flight VA 24.

Authorities said the man became unwell the day he returned, with a dry cough and lethargy, then developed flu-like symptoms a few days later.

He visited the Mussel and Jazz Festival at South Melbourne Market on Saturday, March 7 between 2:00pm and 3:30pm.

He then attended a rugby union match between the Rebels and Lions at AAMI Park, where he sat in section 9.

He has been in home isolation since his diagnosis.

The Department of Health and Human Services (DHHS) said three of the man's close contacts, including two children, were being "closely monitored for signs and symptoms of illness".

Authorities will trace passengers who were in the same or adjacent rows to the man onboard flight VA 24 after they obtain the flight manifest.

Cases speak to activity on US west coast, chief health officer says

The 20th case is a woman in her 20s who authorities said had recently travelled to multiple locations in the US.

She returned to Melbourne via United Airlines flight UA 0060 from San Francisco on February 29 — the same flight as an infected Toorak doctor and the partner of the Carey Baptist Grammar School teacher.

The department said it was contacting the woman's close contacts, who would be told to self-isolate for 14 days.

The three new cases had all travelled in the United States before returning to Melbourne. ( Melbourne Airport, file photo )

The Yeshivah-Beth Rivkah staff member is the 21st confirmed case for the state, and travelled to Melbourne from Los Angeles on Qantas flight QF 94 on March 6.

Another woman who was onboard QF 94 was diagnosed with the virus earlier this week.

"Both [the 20th and 21st cases] have been flights that [had] previously been flagged with confirmed cases," Dr Sutton said.

"I think that speaks to the activity of coronavirus on the west coast of the US now."

Dr Sutton said they were likely to have picked up the virus in the US, rather than on the flight or in Australia.

School and government departments 'mapping potential spread'

The principal of Yeshiva-Beth Rivkah Colleges, Shimon Waronker, told parents in a letter this morning the school would be closed for "the safety and wellbeing of our community".

Some students from the college were being tested for the virus, with results expected today.

Melbourne's Yeshivah-Beth Rivkah Colleges has cancelled classes for a day to respond to the COVID-19 case. ( Wikimedia Commons: Yehoishophot Oliver )

"We are working with DHHS and the Department of Education and Training and going through the process of mapping the potential spread of the virus within the school," Dr Waronker said in a statement.

"We will then make contact with families and identify anyone who could have been impacted from within the school, and where possible, the greater community."

Before the latest case at Carey was confirmed, authorities said eight of the state's cases had recovered, one was in hospital and the remainder were recovering in home isolation.

The Victorian Government has set up a local coronavirus hotline on 1800 675 398.