A late start to the flu season has resulted in an influx of cases into emergency departments and doctors' surgeries over the past month, according to infectious diseases experts.

The flu season usually runs from May to September but this year did not start until the last week in July, said Bill Rawlinson, a medical virologist at Prince of Wales Hospital. A late start to the season means a shorter peak and an earlier finish, he said.

NSW Health officials speculated yesterday that several elderly patients who have died in the past three weeks at the Bethel Aged Care Facility at Waratah, near Newcastle, may have succumbed to flu or another type of respiratory illness.

"Elderly people tend to get the flu worse and they have more complications ... their underlying illnesses and the flu puts more load on their heart," Associate Professor Rawlinson said.

The director of communicable diseases at NSW Health, Jeremy McAnulty, said patients were still undergoing tests at the nursing home. "Nothing has been confirmed - [flu] is one of the strong possibilities or it could be another respiratory illness, or a range of bacteria," Dr McAnulty said.

He confirmed that nine people from the home had been hospitalised and five had died. There had been no reports of influenza outbreaks at other nursing homes, he said.