Experts are baffled over an unprecedented start to the flu season, with more than 10,000 people diagnosed in NSW so far this year — almost three times more than the same period last year.

Key points: At this time in 2018, 3,803 people had been diagnosed in NSW, but this year that number is 10,121

At this time in 2018, 3,803 people had been diagnosed in NSW, but this year that number is 10,121 Nine people have died from the flu in the first four months of the year

Nine people have died from the flu in the first four months of the year NSW Health has 2.5 million flu vaccines at the ready

As NSW Health prepares for the impending peak flu season with 2.5 million vaccinations at the ready, the figures have left health authorities questioning how it will impact the state's health infrastructure.

The figures, revealed in the NSW communicable diseases report, show 10,121 recorded influenza cases to the end of April.

It is almost triple last year's influenza figures for the same period (3,803) and is also almost four times the number recorded in 2017 (2,884).

2017 was a record year for the virus, with 103,852 cases reported.

"We don't know what it means because this is a new phenomenon," NSW Health communicable diseases expert Vicky Sheppard said.

"We don't know if this activity will decrease and then the influenza season will come later in the year or potentially this activity could be ongoing.

"We've had an unusual start."

The NSW figures follow an Australia-wide trend, with influenza figures released in the first week in April showing Australia was nearly halfway towards its 2018 annual total.

Experts predict the flu to kill at least 4,000 people this season.

Between January and March there were seven deaths reported to NSW Health — all associated with influenza outbreaks at aged care facilities.

Another two people died in April.

"We believe that most of the influenza that we saw particularly in January and February was introduced from overseas," Dr Sheppard said.

"And we've got very similar strains to what's been circulating particularly in Europe."

Vaccine at the ready

NSW Health has about 2.5 million vaccines ready to be distributed, with health workers already vaccinating 1.3 million to people across the state.

Dr Sheppard encouraged more people to get vaccinated.

"If you're over 65, pregnant, a child under five or with a medical condition you can get free vaccine from their general practitioners now," she said.

"We've not seen this before — a level of influenza activity so prolonged over the summer months.

"We have more sensitive testing, so we are detecting more cases [but] having 27 aged care facility outbreaks is far beyond what has ever been notified before."

Robert Booy, an international flu expert and head of research at the National Centre of Immunisation Research, said the figures were worrying.

"For this time of year, it's the worst in living memory," he said.

"But there's always the chance it will die down in the next couple of months and we will end up with just an average season."

Professor Booy said hospitals should prepare for a high number of patients.

"We need to make sure we've got resources in place in case emergency departments start getting overly busy," he said.

"We need to prepare for that possibility."