A 10-year-old girl became the country's first case of human-to-human transmission after contracting the disease from a classmate who fell ill upon returning from a family holiday to the United States.

Two teenagers, one in Melbourne and another in Adelaide, were also diagnosed with the virus without having travelled overseas or come into contact with an identified case, prompting Canberra to escalate its pandemic management plan.

Health Minister Nicola Roxon said the shift to the 'containment' phase gave the government greater scope to try to control the spread of the disease.

'It provides us with flexibility for social distancing measures such as school closures, which we have already seen in South Australia and Victoria,' Ms Roxon said. 'The contained phase is not the top phase. There are three more levels after that.'

The alert level was raised following a meeting of the Australian Public Health Committee, and came as five new cases of the virus were confirmed, including the 10-year-old.

A 25-year-old man who flew into Melbourne from Los Angeles on Tuesday was diagnosed with swine flu, while a 17-year-old boy, also from the southern city, was confirmed to be carrying the virus.

A teenage girl from Adelaide became South Australia state's first confirmed case, and her mother, 40, was late Friday found to be suffering swine flu, taking the total number of cases to 12.

Her young brother had tested negative but was displaying flu-like symptoms and would be tested again, health officials said.

Both their schools had been shut for a week as a precaution, and classmates had been prescribed the anti-viral drug Tamiflu. -- AFP

UNKNOWN HOW TEENS CAUGHT FLU

Authorities were trying to ascertain how the Adelaide and Melbourne teens had contracted the disease, Ms 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,' Ms Roxon said.

'There is no cause for alarm but we do need to treat this as a serious matter,' she added.

Australia's chief medical officer Jim Bishop said it was vital the outbreak was brought under control before the country entered the southern hemisphere winter, traditionally flu season. -- AFP