State health authorities are warning people to be vigilant after two cases of monkeypox were confirmed in NSW and Victoria.

The cases - a man in his 40s in NSW and another man in his 30s in Victoria - had both recently travelled to Europe and are now isolating.

The mild illness can be transmitted from person to person through air droplets, close bodily contact or sharing contaminated linens or objects.

Its symptoms include having a fever, headache, muscle aches and exhaustion.

NSW Chief Health Officer Dr Kerry Chant said monkeypox was a rare virus and did not spread easily between people.

She said a large number of overseas cases were among gay and bisexual men and urged those men to "be aware of any unusual rashes or lesions" and to contact their GP or a sexual health clinic if concerned.

Dr Chant said people should be "particularly vigilant" if they had returned to Australia after attending large parties or sex-on-premises venues overseas.

NSW-based sexual health organisation ACON urged gay, bisexual and men who have sex with men to remain vigilant and follow health advice.

"It's important we remember that viruses do not discriminate," said ACON CEO Nicholas Parkhill on Friday.

"We particularly urge those who attended dance parties, sex parties or saunas in Europe to be vigilant for compatible symptoms."

Australia joins Italy, Sweden, the United Kingdom, Spain, Portugal and the United States as nations dealing with recent cases.