Health Minister Brad Hazzard on Monday afternoon said three new cases of COVID-19 had been confirmed in NSW taking the state's total to nine.

Two of the new cases had a "high likelihood of transmission in NSW", the health minister told reporters.

One of the cases was the 41-year-old sister of a man who had returned from Iran with the disease, while the second locally-acquired case was a 53-year-old male health worker who hadn't travelled for many months.

The other new case is a 31-year-old man who flew into Sydney on Saturday from Iran and developed symptoms 24 hours later.

Mr Hazzard said it was "particularly concerning" that the health worker - thought to be a doctor - hadn't travelled for at least three months.

"He's been working in a clinical situation so there is a lot of work to be done ... to determine what contacts he may have had (and) how did he actually get the transmission," the minister said.

Chief health officer Kerry Chant says it's not known if the healthcare worker acquired the infection at work or while in the community.

"Our key focus at the moment is to contact staff or patients that may have been close contacts of this gentleman," Dr Chant said, adding he was in a stable condition in intensive care.

The chief health officer said the patients likely included people aged over 65.

Dr Chant said the fact the healthcare worker was ill raised the question: "Was there a case that was missed?"

"But even if occasional cases are missed, if people practice good hygiene, do common sense things, then their likelihood of onwards transmitting it - even if they are unaware they had it - is much reduced."

Mr Hazzard stressed people should always wash their hands after being in a public space and certainly before touching their faces.

The minister also urged people not to shake hands to reduce the risk of transmission.

"It's time that Aussies actually gave each other a pat on the back for the time being - no hand-shaking," he said.

"It's very automatic but don't do it."

The state government earlier on Monday warned of a likely convergence of a COVID-19 pandemic with winter flu as they announced a lowering of the age at which pharmacists could administer flu jabs to 10.

"While the flu vaccine won't combat COVID-19, it will help reduce the severity and spread of flu, which can lower a person's immunity and make them susceptible to other illnesses," Mr Hazzard said.

The NSW education department has announced that in the current "COVID-19 environment" all overseas excursions in term one are on hold "until further notice".

"Schools have been asked, if possible, to reschedule planned overseas travel until later in the year," the department said in a statement to AAP.

Australian Associated Press