Viral strains predominant in the northern hemisphere in January usually reach Australia by April, causing concern for elderly people and those with chronic medical conditions who rely on antiviral medications to help them beat influenza.

Research from the Centre of Disease Control in the United States shows that about 99 per cent of the most common strains in the north, the H1N1 group, are resistant to Tamiflu. Last year only 11 per cent were resistant.

The rapid pace of the mutation has alarmed virologists, who say it normally takes several years for a virus to develop resistance to a drug.

"We're taking this very seriously," said Robert Booy, a professor in pediatrics with the National Centre for Immunisation Research at the Children's Hospital at Westmead.

H1N1 strain is not usually the most common in Australia. It accounted for less than 0.2 per cent of flu notifications in Australia last year, but it has been the predominant strain in other years.

"H3N2 has been the most common strain in Australia for about 40 years, since the pandemic of 1968, but we have to keep a very close eye on which strains are here this winter in case we have another year where H1N1 is predominant," Professor Booy said.

The centre will decide in April whether to switch from Tamiflu as a frontline treatment to zanamivir, marketed as Relenza, an Australian antiviral with a molecular makeup which makes it harder for the virus to develop resistance. In the US study all the H1N1 viruses were susceptible to zanamivir while all tested influenza A/H3N2 and B viruses were susceptible to both oseltamivir (Tamiflu) and zanamivir.

"We'll carry out our own monitoring to determine the most relevant treatment," Professor Booy said.

NSW Health's director of communicable diseases, Jeremy McNulty, said the mutation had been "quite dramatic" and there was a concern that elderly people, babies and those with chronic medical conditions could be in danger if the H3N2 strain also became drug-resistant.

"This problem really bears watching and is yet another good reason for people to get vaccinated each year to make sure they are resistant to the coming strains," he said. "It's far better to avoid the flu altogether than rely on antivirals, particularly if you are over 65 or Aboriginal and over 50, or you have cardiac or lung problems. Antivirals are not a magic bullet."

A $250 million stockpile of Tamiflu, bought by the Federal Government to combat an outbreak of avian influenza, was still effective and should not be discarded, Professor Booy said.

"So far there is no evidence that H5N1 [the strain which causes bird flu] is resistant to Tamiflu," he said.