Spring has sprung but influenza cases are now only reaching their peak in Australia during a longer flu season than usual.
In the two weeks to September 6, the Australian Respiratory Surveillance report recorded more than 31,000 influenza cases, which is a 27.2 per cent increase from the previous period.
According to the National Notifiable Disease Surveillance Dashboard, there have been 171,440 laboratory-confirmed cases of influenza this year to date
This year, there have been 2,896 confirmed cases recorded in the ACT, 79,376 in NSW, 1,032 in the NT, 33,466 in Queensland, 7,757 in South Australia, 1,672 in Tasmania, 34,552 in Victoria, and 10,689 in Western Australia.
Cases are less than last year but with the season having not yet reached its peak, infectious diseases experts say it is not too late to get vaccinated.
There were several factors contributing to the timing of flu season, University of Queensland virologist Kirsty Short said.
"Some of which are environmental, the circulating strain and the immunity in the population," Professor Short said.
"But what to say is that this seems to be more going back to what we're used to.
"In so far as after COVID, we saw a shift in flu seasonality where our flu peak came earlier and this sort of represents more what we saw pre-COVID with a bit of a later flu peak."
She said it also might be possible people got vaccinated earlier in the year so immunity had waned.
"It's certainly not that immunity completely goes but it does wane over the season and at the start of each season we don't know when the peak will be," she said.
"So it's a really tricky balance to find between the optimal time to get vaccinated, to protect you, but also to ensure longevity of immunity."
Monash University Infectious Diseases expert Allen Cheng said it was a "really strange season".
"Until early to mid-August there had been much less influenza than usual," he said.
"Since the [COVID-19] pandemic, flu epidemiology has been pretty unpredictable so in 2022 it started really early and 2023 also a little bit early."
The most common strain of flu that is circulating is the Super-K subclade of the H3N2 sub-type of Influenza A, Professor Cheng said.
The Super-K flu was the reason for a surge of cases in 2025, which was a record season.
"I do wonder whether there's something about strain K that is just sort of meant that it's more adapted to spreading it this time of year rather than earlier in the normal time of year," Professor Cheng said.
"One theory is that people thought they got it last year, they might not get it this year but we don't know a lot about flu, particularly the role climate change and humidity play."
Professor Cheng and Dr Short said it was still recommended for people to get vaccinated.
"In previous years I would have said at this time of year if you haven't been vaccinated yet I'd say then you are probably not going to be at risk," Professor Cheng said.
"But this year, because the season is still going and with our experience last year, there is still some benefit."
The federal health department recommends Australians aged six months or older receive an annual flu vaccination and discuss this with their doctor.
Australians at higher risk are eligible for free vaccines under the National Immunisation Program.