A man in his 50s who recently returned to Sydney from Queensland is the second person to be diagnosed with monkeypox in NSW and the third in Australia.

The man developed a mild illness several days after he arrived home. He visited his GP before going to hospital, where tests confirmed he had monkeypox, NSW Health said.

It is the second case detected in NSW and is not connected to the first case, which was reported on May 20.

The man is receiving care in hospital.

A man in his 30s tested positive for monkeypox in Victoria after he experienced mild symptoms on May 16.

Another man, in his 40s, was found to be infected after returning from Europe. He was reported as a positive case by NSW Health on May 20.

The latest case lives alone and NSW Health has not identified any high-risk contacts who need to isolate.

Several people who had low-level contact with the man have been told to monitor for symptoms.

NSW chief health officer Kerry Chant said the general population did not need to be concerned by the monkeypox virus, which was rare and did not spread easily from person to person.

"NSW Health is providing further information to clinicians across the state today to assist with the identification and management of potential monkeypox cases," Dr Chant said.

"We will continue to work with GPs, hospitals and sexual health services across the state to provide advice on diagnosis and referral."

NSW Health is working with Queensland Health to identify any possible transmission.

Community health charity ACON has advised men who have sex with men to be vigilant for symptoms.

"It's important we stay informed and continue to be self-aware when it comes to our health," ACON CEO Nicholas Parkhill said.

"We know that people in our communities already have strong health-seeking behaviour when it comes to looking after their sexual health so please continue to monitor for symptoms, including for any unusual rashes or lesions."

Mr Parkhill advised that people keep contact details for their sexual contacts, particularly amid the presence of monkeypox.

"The situation is evolving and being able to reach your sexual contacts should the need arises will help stop the spread," Mr Parkhill said.