NSW Health has confirmed the fifth case of coronavirus in the state.

A man in his 40s who recently returned from a trip to Iran is being treated at Westmead Hospital in western Sydney after experiencing mild symptoms.

NSW Health is urging people who have recently returned from Iran to ensure they are aware of coronavirus symptoms and seek immediate help.

Chief health officer Kerry Chant said this was the fifth person to be diagnosed with coronavirus in Sydney — the previous four have all been discharged from hospital.

"This latest case arrived in Sydney from Iran on 22 February. He did not develop symptoms until 24 February and was seen at a hospital emergency department and tested on 28 February," Dr Chant said.

"He was advised to be isolated at home while waiting for the test result which was confirmed positive for COVID-19 late on 29 February."

Six people were in close contact with the man before his diagnosis and all have since been quarantined, NSW Health said.

Two of these people developed symptoms of mild respiratory illness and have been quarantined at Westmead Hospital while they await test results.

There is nothing to suggest anyone on the same flight as the 40-year-old man is at risk as he became unwell after arriving in Australia, according to NSW Health authorities.

Another man in his 50s in NSW is also under investigation for possible infection and is being cared for in hospital.

The man is the second case in Australia to have come from Iran after a Gold Coast beautician tested positive for COVID-19 late on Friday.

New Zealand's first case also came from someone who had been in Iran.

NSW Health said it was following up on contacts of both the latest confirmed case and the suspected case.

"Travellers from Iran should be particularly vigilant for symptoms as there has been a rapid increase in COVID-19 activity there in recent days," Dr Chant said.

Symptoms include fever, coughs, runny nose and shortness of breath.

It is advised that anyone with symptoms should isolate themselves from others.

Australia now has 25 confirmed cases of coronavirus.

The latest diagnosis comes as a travel ban comes into force today on travellers coming in from Iran.

Any visitors from the Middle Eastern country will have to spend 14 days in a third country before being permitted entry into Australia.

Federal Health Minister Greg Hunt said Australia had been one of the world's leading nations on tackling coronavirus.

"We have acted early, we have acted hard and we have acted decisively," he said on Sunday morning, referring to the travel ban.

"I know there has been some criticism that we may have acted too early on that front. I think the affirmation today is that in acting early, we have been ahead of the curve, but right from the outset we have made it clear that we will take strong action, but that Australia is not immune.

"Therefore we have prepared for all responses."