Australia reported four new cases of swine flu Wednesday, including three young brothers, raising the overall number of confirmed infections to five.
All four had returned from the United States, which has reported more than 5,000 cases, and Health Minister Nicola Roxon said there was no proof of live transmission of the virus within Australia's borders.
"I need to reiterate to the community that we are very well placed to deal with any outbreak of swine flu here in Australia," she told reporters.
A nine-year-old boy returned to Melbourne with his family on a flight from Los Angeles on May 12 and began showing flu-like symptoms this week, Victoria state health officials said.
Two of his brothers, aged 10 and 12, subsequently tested positive for the virus.
Victoria state health authorities quarantined the family, along with classmates of the nine and 10-year-olds.
They said the 12-year-old's classmates did not face similar measures because he had not been at school long enough during his infectious stage to warrant it.
Authorities were also contacting passengers on the flight the family took from the United States as a precaution, Victoria's chief health officer Rosemary Lester said.
She said the school pupils would be quarantined for seven days and treated with the anti-influenza drug Tamiflu.
"Overseas we have seen the disease spread to close contacts, so I think that's certainly a very real possibility," she said.
"But we also need to keep in mind that in developed countries this disease has been quite mild and really not very different in severity to normal seasonal flu."
A 28-year-old Sydney woman was also confirmed as having the virus in the neighbouring state of New South Wales.
NSW Health Minister John Della Bosca said she fell ill while holidaying in the United States and had recovered before she returned home.
US authorities had provided her with a course of Tamiflu and tested her for the swine flu virus, the results of which she had received via telephone this week after returning to Australia.
However, Della Bosca said her travelling companion was also suspected of suffering a mild case of the disease, and all passengers on board their flight were being contacted.
A 28-year-old woman earlier this month became Australia's first confirmed swine flu case after testing positive for a weak strain of the virus upon her return from the United States.
She had been sick overseas and had fully recovered by the time she returned to Australia.
An Australian doctor based in Taiwan became that island's first confirmed swine flu case Wednesday after falling ill on his return from the United States.
The number of swine flu cases worldwide has passed the 10,000 mark, and UN chief Ban Ki-moon has urged people to remain vigilant.
Previous pandemics had shown flu outbreaks could start mild and worsen, Ban told the World Health Organisation's annual assembly in Geneva.
The UN health agency said Wednesday cases had increased by 413 in the past 24 hour with 10,243 infections now in 40 countries, including 80 deaths.