About 1.5 million people in New South Wales could contract the COVID-19 coronavirus in its first wave, according to the state's Chief Medical Officer.

Key points: Those who return from any overseas travel feeling unwell should not attend mass gatherings

Those who return from any overseas travel feeling unwell should not attend mass gatherings Control measures in NSW have not been escalated but the Premier says the situation is evolving

Control measures in NSW have not been escalated but the Premier says the situation is evolving Coronavirus has been declared a pandemic by the World Health Organisation

Dr Kerry Chant said each person who contracted the potentially deadly virus is predicted to infect, on average, at least two other people, based on statistical modelling.

Speaking at a Budget Estimates Committee, Dr Chant said that could result in about 20 per cent of the state's population, or about 1.5 million people, becoming infected in the first wave, according to projections.

"We're nowhere near the peak," she told the committee.

"We've got early evidence of limited community transmission and now is the time we are continuing our endeavours to contract, trace and isolate cases."

Dr Chant said the state's hospitals had been told to double the capacity of their Intensive Care Units and ventilator beds, in order to meet the anticipated burden of presentations due to coronavirus and the flu season.

The number of coronavirus cases in NSW has jumped from 65 to 78 in a day.

NSW Premier Gladys Berejiklian said there will be a further review of mass gatherings across the state.

She asked the public not to panic but to take extra precautions.

"There is no need to massively change what you are doing but it's not business as usual," she said.

"We are asking everybody to be alert."

Overnight the World Health Organisation (WHO) declared coronavirus a pandemic and called on countries to take urgent action because "[we] can still change the course" of the virus.

Dr Chant said the majority of infections in NSW were acquired overseas and 32 cases were contacts of confirmed cases.

But the source of infection for four of the latest 12 cases is not yet known.

"Overnight we have seen cases diagnosed in people without overseas travel, those cases are being thoroughly interviewed to ensure that we can identify any links to other confirmed cases," she said.

Over 9,000 people in NSW have been tested for COVID-19 which Dr Chant described as "reassuring".

High test numbers mean further spread will be restricted, she said.

"If you look at the testing numbers in some countries overseas you can see that they've actually had very, very low rates of testing and hence the spread of the virus can become unrecognised until you're dealing with deaths," she said.

The Premier said control measures have not had to be escalated in NSW and the majority of people who may contract the virus would make a full recovery at home.

However she said the Government was on stand-by as the situation was evolving.

Dr Chant said anyone who felt unwell, irrespective of overseas travel, should "socially distance" themselves and stay clear of mass gatherings.

She also urged those with flu-like symptoms to stay home from work.

"The best thing you can do is stay at home if you're unwell — don't push through, don't go to workplaces, don't go and visit loved ones in aged care facilities, don't go and visit loved ones in hospital and don't go to public events where you're going to be cheek and jowl," Dr Chant said.

Any bans on mass gatherings or school closures were a matter for Council of Australian Governments (COAG) health ministers, she said.

NSW Health officials will provide more information on the movements of the four unlinked cases throughout the day.