Both women were diagnosed after returning from a trip to the United States and have been treated with the antiviral drug Tamiflu.

Health authorities are testing 13 people from NSW and other states who were travelling with them.

Their cases were announced minutes after a nine-year-old boy from Melbourne was listed as Australia's second victim.

Victorian Health Minister Daniel Andrews said he was recovering at home and his parents and siblings were being treated with Tamiflu.

"The child's symptoms appeared on Monday and, based on what we know about this form of influenza, agencies do believe he was not infectious when he returned to Australia on May 12," Mr Andrews said.

Victoria's acting Chief Health Officer Rosemary Lester said the test results from the rest of the family were expected during the next few days.

She said other precautions included contacting other passengers on flight QF94 from Los Angeles to Melbourne on May 12 who were sitting close to the family.

"Based on what we know about this form of influenza, passengers on the flight are highly unlikely to become unwell because we strongly believe that the boy was not infectious when he travelled,'' Dr Lester said.

She said any other passenger of the flight with concerns could call the Swine Influenza Hotline on 180 2007.

Australia's first case was a 28-year-old NSW woman who flew into Brisbane from Los Angeles on May 7.

The woman was no longer infectious and had a weak strain of the virus.

- with AAP