Epidemiologists are warning Australians that a second wave of swine flu is likely to sweep through workplaces, homes and classrooms.

They say it is only avoidable if people vaccinate themselves against the virus.

Across the Northern Hemisphere the swine flu returned with the winter cold, and according to the World Health Organisation, it claimed thousands of lives.

But Professor Robert Booy, from the Westmead Children's Hospital, says this can be prevented from happening in Australia.

"Never before have we been in this position where we had a highly effective and safe vaccine so that we could intervene and stop that second wave," he said.

"The Northern Hemisphere has already suffered their second wave, but we still can make a difference."

Health professionals say many Australians who have returned from trips to the Northern Hemisphere during January have brought the virus back with them.

And now children are going back to school, increasing the risk of spreading the virus.

But so far the uptake of the flu vaccine has been far less than the Federal Government hoped or expected, perhaps because in most cases the swine flu is a mild illness.

Professor Booy says it is important people with pre-existing medical conditions get immunised because the disease may pose a greater threat to them.

Professor Alan Hampson, the chairman of the Influenza Specialist Group, says it would be a mistake for others to underestimate the potential dangers of the illness.

"A lot of people - about 30 per cent of people who were seriously ill and died - were people who didn't have any known risk conditions," he said.

"It's still a disease which has had a very substantial impact and it's still a disease which really does need dealing with."

Dr Booy says it is also important that healthy people, and particularly children, get immunised in order to help protect those who are more vulnerable.

"We've seen much higher rates of hospitalisation in young children and so there's two reasons to vaccinate them," he said.

"First of all, because they're at risk of becoming so ill as to need hospitalisation, but secondly, because they're the great transmitters.

"They're the ones that spread the infection so widely and quickly in the community"