The number of coronavirus cases in New South Wales, Australia's hardest-hit state, has jumped to 77 overnight after the World Health Organization formally declared a pandemic.

While the majority of confirmed cases were contracted overseas or had close relationships to other people with COVID-19, NSW Chief Health Officer Kerry Chant warned that there were a number of cases in the Ryde-Hornsby area of Sydney where authorities were unable to determine the source of the virus.

Some of the people diagnosed overnight had not recently travelled overseas, Dr Chant said, but additional details of their movements were not provided as of Thursday morning.

"Now is a time for caution," she said at a press conference following the WHO declaration on Thursday morning. "If you are unwell, don't go to a public gathering and spread your germs at this time."

Premier Gladys Berejiklian said the virus was evolving and it should not be treated as "business as usual".

"I also want to let our citizens know there is no need to panic. There's no need to massively change what you are doing, but it is not business as usual," she said.

"What you hear today might be different from tomorrow, might be different the next day. We are asking everybody to be alert."

Ms Berejikian said the government would continue to review their policies on mass-gatherings and major events following the cancellation of the Dark Mofo festival in Tasmania and Coachella music festival in the United States.

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"I want to assure all of our citizens that if you take the precautions and you're extra careful, that there is nothing to be concerned about," Ms Berejiklian continued.

"The vast majority of people, the overwhelming majority of people who even contract the virus, can be treated at home safely and don't ... need extreme medical intervention any stretch."

There have been at least 128 confirmed cases of COVID-19 across Australia and three deaths, and 24 of these cases are reported to have recovered.

Three people have died, including two elderly residents of the Dorothy Henderson Lodge Aged Care Facility in Sydney's north and a 78-year-old passenger of the Diamond Princess Cruise Ship who died in Perth.

Earlier on Thursday, WHO chief Tedros Adhanom Ghebreyesus announced that the COVID-19 outbreak had officially become a pandemic after it reached more than 100 countries.

READ MORE World Health Organization declares coronavirus outbreak as a pandemic

More than 122,000 cases have so far been confirmed across the world, with at least 66,600 recovered and approximately 4,500 deaths.

Dr Chant on Thursday once again urged the community to follow official health advice, including not going to work or visiting aged care or medical facilities if unwell.

Coronavirus symptoms can range from mild illness to pneumonia, according to the federal government's website, and can include a fever, coughing, sore throat, fatigue and shortness of breath.

People who believe they may have contracted the virus are advised to call their doctor before organising an appointment or contact the national Coronavirus Health Information Hotline on 1800 020 080.