Nearly 21,000 cases of flu have been confirmed in Australia so far this year, double the number of cases at this time last year.

There has been a sharp increase in the number of cases of flu this year.

Nearly 21,000 cases of influenza have been confirmed so far in Australia, double the number of cases at this time last year, the Influenza Specialist Group (ISG) reports.

Swine flu, or H1N1, is the most common influenza virus type, it says.

The ISG says around 20 to 50 per cent of children are likely to contract the flu each season, compared with 10 to 30 per cent of adults.

And women are 25 per cent more likely than men to contract the flu.

ISG said this is not surprising as only about 30 per cent of Australia women are vaccinated against the flu.

As a fit and healthy 40-year-old, Gai O'Dwyer never thought she'd be vulnerable to influenza.

But the Gold Coast mum recently came down with the flu and it took about six weeks for her to recover.

"I just had no idea that the flu could do this to me," said Ms O'Dwyer.

"I'm fit, I'm young and healthy. I very rarely get sick.

"Had I known I could have prevented all this, I would have been vaccinated a long time ago."

As of August 11, there have been 20,979 cases of laboratory confirmed influenza reported, according to the Department of Health and Ageing Influenza Surveillance Report and Activity Updates.

The majority of notifications have been in Queensland with 5959 cases.

NSW has had 5375 cases, Victoria 2473, SA 1797, WA 1722, NT 331, ACT 284 and Tasmania has had 197 people test positive for the flu.

ISG chairman Dr Alan Hampson said one of the reasons Queensland has the highest number of cases may be because they test more.

"Queensland tends to test more than the other states so that might be a reason for higher numbers, and a warmer climate doesn't protect people from flu," said Dr Hampson.

He said people tend to think of the flu as an illness that has its worst impact on the elderly.

But he noted the median age of patients admitted to hospital for the flu is 42 years.

"Many of those seriously affected are not elderly people. They are regular working people," he said.

Dr Hampson said vaccination remained the best protection against the virus, adding that it's late but not too late to get vaccinated.

"Vaccination is the single best protection," Dr Hampson said.

"With vaccination, you are protecting yourself and those around you from serious illness.

"But don't put it off. The window is rapidly closing because your body needs time to respond to the vaccine."

The flu vaccine is free for pregnant women, those aged over 65 years, Indigenous Australians 15 years or older, and children and adults suffering underlying medical conditions.