Sixteen children have been admitted to intensive care since May with "life-threatening complications" from influenza at three NSW hospitals, including John Hunter.
NSW Health Chief Health Officer Dr Kerry Chant said these complications include "serious heart, brain, and muscle-related complications".
Four affected children were treated at John Hunter Hospital, physician Dr David Durrheim confirmed.
The others were treated at the ICUs at Sydney Children's Hospital at Randwick and The Children's Hospital at Westmead.
Dr Durrheim said the Hunter New England district had 94 emergency department presentations with flu-like illness across all ages in the past week.
"That is very high for this time of year," Dr Durrheim said, adding it was early in the normal flu season.
In the same week in 2018, the district had less than 10 emergency department presentations with flu-like illness. In 2019, there were 26 cases.
Data from the past five years showed the district hit a high of 140 emergency department presentations in a week for flu-like illness in mid-August 2018.
The number of confirmed flu cases this year highlighted the extent of the early-season surge.
"In the year to date, we've had 2710 confirmed flu cases in Hunter New England," Dr Durrheim said.
"Before the pandemic in 2019, that was quite a big flu year, it was 1762 in the year to date."
Although, he said there may be more testing now because "people are more attuned to COVID".
Dr Durrheim said COVID hospitalisations had "dropped to the high 30s, which is big change from a month ago when they were in the 90s".
"That's a good sign that COVID activity is going down. Unfortunately in its wake we have flu activity surging."
Dr Chant said children were being "disproportionately affected by influenza this year" and numbers were "continuing to rise".
Dr Durrheim said the influenza A and B viruses were "on the way up unfortunately, which is unusual".
"We normally get a little flu B often in young children at the end of the flu season in September. This is unseasonal.
"That probably explains some of the increase in emergency department presentations and the admissions to ICUs and hospitals."
He said the flu was about "three times as common at the moment" as COVID in people with respiratory illness that did PCR tests.
Dr Durrheim urged people to get the flu vaccine.
"The good news is the vaccine is a good match, so it should provide good protection in the high risk groups," he said.
"It would be really smart for parents to get their children jabbed. If you can keep your kid out of ICU, what a jolly good thing to do.
"Children aged six months to five can get the jab free."
For those who have to pay, he said the flu vaccine is "quite affordable and convenient".
"It would be worth popping into a GP or pharmacist and getting a jab."
Dr Chant also urged parents to book their kids in for a vaccine during the school holidays.
