Three boys from the same family in Melbourne are among four new confirmed cases of swine flu in Australia.
Health officials say the children and a New South Wales woman contracted the virus while travelling in the United States.
A fifth person, the woman's travelling companion, has displayed flu-like symptoms and is being tested.
Two of the boys are students at Clifton Hill Primary School, in Melbourne's inner north, where their classmates are being treated with Tamiflu. They will all spend a week at home in quarantine.
Australia's first case of swine flu was confirmed earlier in May, when a woman tested positive after returning from Los Angeles. She has since recovered.
Meanwhile, Taiwan's health officials have reported the island's first case - a male foreign national.
Swine flu has killed 81 people and been confirmed in nearly 10,000 people globally.
The vast majority of cases are in Mexico and the United States, but the spread of the disease to about 40 countries, including New Zealand, led the World Health Organisation to declare a pandemic was imminent at the end of April. Its pandemic alert level is now 5 on a 6-level scale.
WHO director-general Margaret Chan said the outbreak that began in North America and has stretched to Europe, Asia and South America needed to be tackled with seriousness even though its symptoms appear to be largely mild.
"For the first time in humanity, we are seeing, or we may be seeing, pandemic influenza evolving in front of our eyes," she told the WHO's annual congress in Geneva.
Japan on Monday reported 125 swine flu cases and closed more than 2,000 schools and kindergartens in a bid to slow the spread of the virus.
The number of cases has risen rapidly in the urban areas of Kobe and Osaka since the first confirmed domestic infection, a 17-year-old Kobe high school student who had not been abroad, was reported on Saturday.
New Zealand's deputy director of public health Fran McGrath says there is no indication the virus is spreading in Japan beyond two educational institutions and there is no plan to increase border controls in New Zealand.
