The figures already exceeded 2024's record 365,000 case numbers and were driven largely by slumping vaccination rates, college president Michael Wright said.
"This is not a record we want to be breaking," Dr Wright said in a statement.
"We must boost vaccination rates and reverse this trend."Â
The college said 25.7 per cent of those aged six months to five years were vaccinated in 2025 - the lowest rate since 2021.
Vaccination rates were also down among those older than 65, with the cohort's 60.5 per cent rate the lowest since 2020.
Cases continued to climb, with more than 13,000 confirmed so far in October - an increase of almost 50 per cent on the previous corresponding period, the college said.
Griffith University professor Paul Buynder said sagging vaccination rates and increased international travel by Australians were pushing up the number of flu cases.
"There's a lot more inter-seasonal influenza these days with more people travelling to the northern hemisphere," he told AAP.
The COVID-19 pandemic had also caused mistrust among some people about vaccinations.
There was "despair about whether the flu vaccine or any vaccine works or whether they can be bothered getting a vaccine", Prof Buynder said.
In September, health authorities said parents in Queensland and NSW would be able to access a nasal-spray vaccination ahead of the next flu season to protect children from the illness and boost herd immunity.
The Australian Medical Association said earlier this year the drop in flu jab rates and COVID-19 booster shots was alarming.
It said an annual influenza shot provided strong protection, with more than 98 per cent of 2025's circulating strains matching vaccine components.
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