Health Minister Brad Hazzard has extended the winter season free jab program for all state residents by two weeks to July 17, or until the end of the two school week holiday period.
"It only takes a few minutes to get a flu jab but that time could mean the difference between you or one of your loved ones ending up in ICU so please, book in today," Mr Hazzard said.
Just one-in-four children under five years have had a flu shot, and the number is even lower for kids aged five to 15.
"It's really worrying that just over 18 per cent of children and teenagers have had a flu jab and for kids aged six months to five years, the figure is 25 per cent," Mr Hazzard said on Thursday.
"These numbers are particularly concerning given in the last month, four times as many kids have been admitted to Sydney's two children's hospitals with flu than with COVID."
Fewer than 40 per cent of 50-to 65-year-olds and only 64 per cent of those aged over 65 have had a flu jab.
Chief Health Officer Dr Kerry Chant says flu vaccinations prevent unnecessary hospitalisations.
"More than 1000 people presented to our EDs (emergency departments) with flu-like illness last week and almost 165 were so unwell they were admitted, including very young children," he said.
Authorities are concerned about low flu jab rates in Sydney's west and southwest as well as parts of far northern NSW and the central west.
