With a 47 per cent rise in flu cases in May compared with the same month last year according to federal health department figures, the worst is yet to come.

"This year has all the hallmarks of a severe season," said Influenza Specialist Group chairman Alan Hampson.

"We're just starting to see the beginning of the rise in cases, and we expect that to peak around August, September," Dr Hampson said.

Dr Hampson said the total number of people affected could be in the low millions.

"That's going to run into the many hundreds of thousands of Australians who will contract influenza, possibly getting into the low millions ... in a bad year, it may get to 15 or 20 per cent of the population infected," Dr Hampson said.

The season is also expected to overload emergency departments as the elderly contract the illness and move from aged care to hospitals, filling hospital beds. Children and pregnant women are also vulnerable to complications resulting from the flu.

"When flu surges, it does for about six weeks, and it makes it much busier in emergency. We could have up to 200 children admitted to our hospital in that six-week period," said infectious disease expert at the University of Sydney, Robert Booy.

NSW Health says that while more flu cases have been recorded so far this year than in previous years, it doesn't indicate a more severe flu season to come.

"The increased numbers of flu cases have occurred before winter and didn't result in widespread community transmission," a NSW Health spokesperson said.

An increase in cases could also be due to more people being tested for influenza infection, rather than the number of flu cases actually increasing.

"Since the influenza pandemic in 2009 there has been an increased use of virological testing in primary care, which has contributed to the higher number of notified cases in more recent years," said communicable diseases director of NSW Health, Dr Vicky Sheppeard.

The department has a plan to handle a possible surge, including using paramedics who can treat patients in their home rather than hospital and clinical support of aged care facilities.

And though the rise in cases this early in the year is out of the ordinary, experts say it's not the result of the federal government's late rollout of the 2015 vaccine.

They say that overseas travellers bringing in the flu from a bad season in the northern hemisphere accounted for higher-than-usual flu numbers in the first half of 2015.

They also urge people to get vaccinated, saying it is not too late to be protected against flu.

"If we're in for a flu season, you can still catch the flu late in winter and in early spring. The most effective prevention strategy is still a vaccination," said UNSW infection control expert Mary-Louise McLaws.

She said that it was extremely difficult to control the spread of the flu otherwise, though vigilant hand washing, cough etiquette, and staying home from work or school once sick can prevent the spread.

"We have found from our research that influenza particles can be spread just by talking and breathing. If someone is incubating influenza they're most infectious two to three days before their symptoms occur," Professor McLaws said.

Each year between 1500 and 3000 Australians die after getting the flu, about 18,000 people are hospitalised, and 300,000 visit a doctor.