A Victorian man is in hospital after being diagnosed with monkeypox, while another in NSW is also suspected of being infected with the virus.
Victorian chief health officer Dr Brett Sutton said the man travelled from London, via Abu Dhabi, arriving home on Monday.
He said the man had developed symptoms prior to boarding the flight, then sought medical care almost immediately after arriving home.
"[He] had an extremely astute GP, who has thought of monkeypox and referred him for testing, which has led to early diagnosis, early isolation," Sutton told reporters.
The man is in a stable condition at The Alfred Hospital and is expected to make a full recovery.
"There are very few close contacts that have been identified. Obviously, the GP is one of them," Sutton said.
"We'll be contacting individuals who are close contacts on the flight — so really in a couple of rows in front and behind — and then people who are on the entire flight, we are just asking to be aware of the symptoms that might occur.
"Those symptoms are flu-like symptoms — they can be headache, fever, muscle aches and pains."
Sutton noted there would be people in Australia born after 1980 who would not be vaccinated against smallpox, which would protect them from monkeypox.
He said Australia should expect to see more cases emerge but did not expect border closures.
Meanwhile, NSW Health said a man aged in his 40s was another "probable case", saying he had recently returned to Sydney from Europe.
He developed a mild illness several days after arriving home, then went to his GP with symptoms clinically compatible with monkeypox.
Urgent testing identified a probable case of monkeypox, with confirmatory testing under way.
He and his household contact are isolating at home.
NSW chief health officer Dr Kerry Chant said authorities had issued a clinician alert to GPs and hospitals across the state.
"[NSW Health] has also been in contact with sexual health services to increase awareness of the cases identified overseas and to provide advice on diagnosis and referral," she said.
"We will be speaking with GPs about this issue again today."
Monkeypox is a rare viral infection that does not spread easily between people and is usually linked to travel to central or west Africa.
"Cases are occasionally reported in non-endemic countries in returning travellers or their close contacts, or in owners of imported pets," Chant said.
"People can contract monkeypox through very close contact with people who are infected with the virus.
"The infection is usually a mild illness and most people recover within a few weeks."
What is monkeypox?
Monkeypox is a virus that originates in wild animals like rodents and primates and occasionally jumps to people. Most human cases have been in central and west Africa, where the disease is endemic.
The illness was first identified by scientists in 1958 when there were two outbreaks of a "pox-like" disease in research monkeys - thus the name monkeypox. The first known human infection was in 1970, in a 9-year-old boy in a remote part of Congo.
Monkeypox belongs to the same virus family as smallpox but causes milder symptoms.
Most patients only experience fever, body aches, chills and fatigue. People with more severe illnesses may develop a rash and lesions on the face and hands that can spread to other parts of the body.
The incubation period is from about five days to three weeks. Most people recover within about two to four weeks without needing to be hospitalised.
Monkeypox can be fatal for up to one in 10 people and is thought to be more severe in children.
People exposed to the virus are often given one of several smallpox vaccines, which have been shown to be effective against monkeypox. Anti-viral drugs are also being developed.
On Thursday, the European Centre for Disease Prevention and Control recommended all suspected cases be isolated and that high-risk contacts be offered the smallpox vaccine.
It's the first time monkeypox appears to be spreading among people who didn't travel to Africa. Most of the cases involve men who have had sex with men.
In Europe, infections have been reported in Britain, Italy, Portugal, Spain and Sweden.
Britain's Health Security Agency said its cases are not all connected, suggesting that there are multiple chains of transmission happening. The infections in Portugal were picked up at a sexual health clinic, where the men sought help for lesions on their genitals.
On Wednesday, US officials reported a case of monkeypox in a man who had recently travelled to Canada, where authorities are investigating suspected infections in the Montreal area.