Australia has seen a resurgence of the H1N1 virus, or swine flu, with the virus accounting for the majority of reported influenza cases this year, experts have said.
The Influenza Specialist Group (ISG), which monitors the spread of influenza, said there had been more than 21,000 reported cases of influenza so far this year, up from about 15,400 this time last year.
Almost three quarters of the reported influenza cases for 2014 were the H1N1 strain, sample figures from the World Health Organisation showed.
ISG chairman Dr Alan Hampson said the latest outbreak of swine flu was the main reason this year's flu season was so severe.
"It depends on the strains of virus, climatic conditions, all sorts of things. This year we've got a return of what we call the swine flu, that we first experienced in 2009," Dr Hampson said.
"It came back into the northern hemisphere this last winter, particularly in North America, and they had quite a severe outbreak there.
"That's the predominate virus, but not the only one this year."
Vic flu season catching up with Qld, NSW: Doctor
Queensland has reported the highest number of flu cases in the country so far this year, with more than 6,800 cases - about 200 more than New South Wales.
But Dr Hampson said the flu season had not reached its peak in Victoria and expected the number to spike in the coming weeks.
"The numbers are increasing. New South Wales and Queensland are well ahead of us, but my guess is the peak will occur somewhere in the next four to eight weeks in Victoria," he said.
"Queensland for some reason always seems to record fairly high levels, but New South Wales doesn't normally."
Meanwhile, Royal Children's Hospital Brisbane infection specialist Dr Michael Nissen said young mums were most at risk.
"Children have a higher rate of influenza and also they're more likely to spread influenza. We also know a lot of mothers catch flu from their children," Dr Nissen said.
Dr Nissen said people should have the flu needle and maintain good hygiene.
"Australia-wide we have 21,000 cases confirmed compared to last year and the worrying thing is that over 8,000 of those are actually in young adults aged between 20 to 49 years of age."
'Not too late' for flu vaccination
Dr Hampson said keeping your hands clean, particularly when using things like shopping trolleys and door handles, and avoiding big crowds was the best way to avoid getting sick.
He also played down the use of over-the-counter medicine to try to fight off the illness.
"They're not really having an impact in the infection in your body, they're just suppressing the symptoms," he said.
"So you really shouldn't take those and then soldier on. It only stresses your body."
Dr Hampson said it was not too late to get vaccinated against the flu, but it may not be as effective this late in the year.
"If they haven't been infected yet it's never too late but there might be a very narrow window," he said.
"I think people do need a while to acquire immunity."If they're in a high-risk group or people who can't afford to be infected then it's worth while getting along to your GP and I'd suggest no later than this week."