About 300 people have died from the flu already this year - more than double that in all of 2018.

But experts believe Australia's "moderately bad" season has probably hit its peak.

The flu season started early this year after a mild one in 2018, leading to significantly higher numbers of influenza cases than usually experienced by this point.

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Influenza expert Professor Robert Booy believes it will be a moderate flu season and not as severe as in 2017, when 1163 died.

The Immunisation Coalition chairman said the early start to this year's flu season will probably mean an early peak, predicting the number of cases will begin to decline soon.

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"My view is that it may be a moderately bad year with an early onset, but I can see evidence that it's peaking, that it's plateauing and it's likely to start falling much sooner than usual," he told AAP.

Prof Booy, from the National Centre for Immunisation Research and Surveillance, said the flu season often peaked in August and September but he believed it had already peaked.

'Moderate' flu season

NSW Health's director of communicable disease Vicky Sheppeard also described this flu season as moderate, saying the unprecedented numbers were due to the early start and followed an unusual amount of activity over summer.

"Flu is an unpredictable virus, but we're hoping that we're at the peak and in the coming weeks we will start to see a reduction in activity," Dr Sheppeard told reporters in Sydney on Thursday.

There have been 135,952 laboratory-confirmed cases of influenza across Australia so far this year, well above the 17,349 average at the same point over the previous five years, federal health department data shows.

Deaths doubled

So far 298 deaths associated with influenza have been reported to the National Notifiable Diseases Surveillance System, although that only includes laboratory-confirmed cases.

There were 58,847 confirmed influenza cases and 125 deaths in Australia in 2018.

The 2017 influenza season, when a quarter of a million laboratory-confirmed cases were recorded and 1163 people died, was the worst since the 2009 swine flu pandemic.

The laboratory numbers do not show the full extent of the flu because most people do not get tested.

Mutating strains

Prof Booy said modelling estimated the number of deaths on average each season was actually about 3000 to 4000, and the Immunisation Coalition expected this year would be no different.

While there have been reports of one of the strains of flu mutating, Prof Booy said a mutation of the virus was a natural event and the World Health Organisation Influenza Centre in Melbourne was not reporting any mutation that meant the vaccine did not work.

Dr Sheppeard said the WHO had noted a minority of one of the four circulating strains of flu carried a mutation that made it a bit different to the vaccine, but the vaccine remained strongly effective against most of the circulating strains.

'As well as expected'

She said the vaccine appeared to be working as well as expected, preventing about half of potential flu cases and reducing its severity for sufferers.

"Unfortunately, particularly in people 65 and over, despite vaccination we do get fatalities each year."

Dr Sheppeard said WHO had also advised that there was no antiviral resistance in the strains of flu circulating at the moment.

Alarming figures

REPORTED CASES OF LABORATORY-CONFIRMED FLU AND ASSOCIATED DEATHS SO FAR IN 2019

135,952 cases nationally

298 deaths

Source: Department of Health (federal)

LATEST AVAILABLE STATE-BY-STATE DATA**

SA - 19,964 cases, 82 deaths

NSW - about 43,000 cases, 70 deaths

VIC - 25,969 cases, 50 deaths

WA - 17,640 cases, 48 deaths

QLD - 23,947 cases, 38 deaths

ACT - 1595 cases, fewer than 5 deaths

NT - 1079 cases, 4 deaths

TAS - 1390 cases, 1 death

Jurisdictions update their data at different times.