At the same time last year health authorities had been notified of just 124 cases. And from January to May 2017 there was 203 notifications - ahead of one of the worst flu seasons on record.

ISLHD co-director of critical care Dr Simon Keane said the early spike in figures was not always indicative of a massive flu season to come - but it was a good idea to take precautions.

"We are seeing higher numbers of people with influenza A - the predominant strain locally and across the state - coming to the emergency department earlier in the season than would be expected," he said.

"Our hope is that the flu season has come early and will now trail off, however there is the concern that it could lead to a significant flu season.

"So we're trying to get the message out that the best thing is to prevent illness in the first place, and that is by getting vaccinated."

Dr Keane also urged generally healthy people with flu symptoms to prevent the spread of infection by staying home from work, not visiting vulnerable people and only going to the emergency department in an emergency.

"Influenza is a nasty viral infection," he said. "And while a healthy person will get a cough, fever, running nose and feel terrible for a week or two, it will usually run its course and they'll get better.

"But for other patients who are high risk - such as those with heart or lung disease, the very young and over 65 - then it can have really significant and life-threatening consequences.

"That's why we'd ask generally healthy people with flu symptoms to stay away from the ED if possible, so they're not spreading the infection to high risk patients coming through emergency."

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However those people who were experiencing severe symptoms such as shortness of breath, should seek emergency attention.

"If your flu symptoms are getting worse, for instance if you have difficulty breathing or shortness of breath, then you need to come into the ED where we can assess you and escalate treatment as appropriate," Dr Keane said.

"While you're there we will put measures in place to limit the spread of infection - by giving you a mask or placing you in an area away from other patients."

Simple prevention for those in the community includes practising good hand hygiene, and sneezing or coughing into your elbow rather than your hands.

It's been an early start to the season across NSW, with 13,888 confirmed flu cases as of May 24. Peak flu season is usually from July to September.