In the past week, 111 people sought treatment for flu at hospital emergency departments across the state, more than double the number for the same period last year. Thousands more each week are seeking relief from their GPs.
Experts are puzzled as to why this season's flu is so severe. "There is a significant amount of influenza activity and an increase in the number of cases presenting to emergency and in laboratories this year," says Dr Dominic Dwyer, a medical virologist from Westmead Hospital. "It has been the worst we've had in several years."
Dr Dwyer says it is difficult to know why so many people were falling victim to influenza viruses, the symptoms of which include fever, headache, sore throat and severe tiredness.
"It's hard to know whether this is due to a particularly virulent strain, but certainly there have been some deaths in the past few weeks."
Older people and those with low immunity were more likely to be among those worst affected, he says. Dr Dwyer said it was not too late for people to be vaccinated against flu, but it should be done as soon as possible as it generally took two weeks after the injection for the body to develop immunity.
The most common respiratory viruses identified in hospitals this season are influenza A and respiratory syncytial virus.
The senior medical virologist at the Prince of Wales Hospital, Professor Bill Rawlinson, said the hospital was mainly seeing a virus known as A/Wisconsin-like virus, which is present in the flu vaccine. "We hope the vaccine is protecting well and that people are getting it," he said.
"The reality is if you do get the flu, you are often out for two weeks and even if you have had the injection it can [only] give you partial protection."
Professor Rawlinson said the degree of virus mutation often dictated how bad the flu season would be. Research shows that people are more susceptible to the flu in winter when there is less sunlight to produce immune-strengthening melatonin.
Professor Rawlinson advises those with the flu: "Take it easy, don't go to work and spread it to anybody else."