Public health physician Dr David Durrheim said the Sydney outbreaks were "a real reason for concern" as NSW Health reported another 20 cases of coronavirus in the 24 hours to 8pm on July 19.

"We would strongly advise anybody from the Hunter New England region to rather stay at home than visit Sydney at this stage unless absolutely necessary," Dr Durrheim said.

"This virus, if you give it half a chance, can spread very, very rapidly and even from asymptomatic people in the pre-symptomatic stage."

It comes as another Hunter resident tested positive for coronavirus while in hotel quarantine in Sydney.

The region's latest confirmed case is a 34-year-old man who recently returned from the Middle East.

Dr Durrheim said it was important that people get tested - and re-tested - for COVID-19 when symptoms appear.

"If you have been tested in the past, a single negative test does not mean if you develop new symptoms that you are COVID-free," he said.

"It's very important to again go and get yourself tested.

"The good news is that the new testing regime, which includes a deep nasal swab rather than a nasal-pharyngeal swab is much more comfortable.

"We encourage anybody who develops even mild symptoms - a scratchy throat, a runny nose, a mild fever, a cough - get yourself tested and make sure you don't have COVID-19. Let's make sure we keep our community safe against this virus.

"Our only defence is really standing back and making sure that we take real care with our hand washing and are not taking coughs and colds into the community and spreading the virus."

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