Australian authorities were attempting Friday to trace the movements of two people confirmed to have contracted swine flu without having travelled overseas, as the number of cases here hit 11.
Four new cases of the virus were confirmed overnight and on Friday, including a 10-year-old girl who was a classmate of a Melbourne boy who fell ill, along with his two brothers, after returning from a US holiday last week.
National Health Minister Nicola Roxon said it was the country's first case of human-to-human transmission, and marked an important change in the disease's status.
"We do seem to be moving into a new phase of more widespread transmission in Australia," she said.
A 25-year-old man who flew into Melbourne from Los Angeles Tuesday had also been diagnosed with swine flu, while a 17-year-old boy, also from the southern city, was confirmed to be carrying the virus.
A teenaged girl from Adelaide became South Australia state's first confirmed case, and authorities were trying to ascertain how she and the Melbourne teen had contracted the disease, Roxon said.
Neither had been overseas or had known contact with identified cases, and intensive tracing of their movements was now under way, she said.
"Clearly these developments overnight are a cause for concern, particularly the first known case of human to human transmission and, of course, now two cases ... that don't have any history of travel or contact with confirmed cases," Roxon said.
"There is no cause for alarm but we do need to treat this as a serious matter," she added.
Authorities have warned community transmission of the highly infectious influenza was "inevitable," and Roxon said the confirmed cases could rise on an almost hourly basis.
Alan Hampson, a consultant to the World Health Organization, said there was a high probability the virus would continue to spread, but Australia's pandemic plan did not yet need to move from the "delay" to "containment" phase.
Such a shift would allow for the closure of schools and other public places such as cinemas and nightclubs and cancellation of major sporting events.
"Containment would be in the situation where we actually do see community-based transmission and certainly we're far from that at the moment," Hampson told state radio.
"That's not to say it won't happen at some stage down the track, but right at the moment, (there is) a very small number of cases in Australia."
More than 11,000 cases and 85 deaths have been recorded since the outbreak of A(H1N1) influenza emerged in Mexico and the United States a month ago, and the world remains at flu alert level five, signalling an "imminent pandemic."