Flu cases this year are occurring at a rate three to four times higher than normal.

Nationwide, experts predict about 4,000 people will die from complications due to influenza this season.

There have been over 13,000 confirmed cases of the flu and 26 deaths in Victoria alone this year.

However, the World Health Organisation says those figures do not show the true extent of the problem.

Ian Barr, deputy director of the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, says the state's reporting process is a “piecemeal analysis”, and real figures are much worse.

“The 13,000 cases, [you] could easily multiply it by 10, no-one really knows,” Professor Barr said.

He said the state only measures the number of people who get tested or die from the disease.

“Both of them are only tips of the iceberg guestimates,” he said.

“There's no accurate figures on this but you can imagine something like only 10 per cent of people would bother going to the doctor and that doctor having a sample taken and having a result done.

“Probably the best estimates you can get is looking at the deaths in the US that happen per year and dividing that by our population, so dividing their numbers by 10, something like that.

“[In] the US, they estimate [there are] between 36,000 and 61,000 deaths per year from influenza.

“So if you divide that by 10 it probably gives you an idea of somewhere around the numbers who die of flu each year [in Australia].”

The experts suggest hospitals may consider year-round testing regimes.

Immunisation Coalition chairman Prof Robert Booy says there are more and more cases emerging in summer and autumn.

“The best explanation is that 2018 was so quiet that we have reduced community immunity, so there are more people who are vulnerable to catching infection and therefore transmitting infection,” he said.

While ABS statistics showed 1,200 influenza deaths in 2017, Dr Booy said modelling showed the actual number of deaths was closer to 3,000.