An early spike in flu cases has prompted a vaccination push from health authorities.
More than 4700 people have been diagnosed with influenza in the past four weeks in NSW, a 16 per cent increase compared with 2023.
And hospital admissions from influenza-like illness are nearly double that of the same period last year.
Nine children aged under 16 were among 39 people who died from the flu in Australia in 2023.
It marked a rise in the number of flu-associated deaths among children from 2022 and many pre-pandemic years.
"Last year, we saw an increase in [emergency department] presentations among young children with some being admitted to intensive care with life-threatening complications from the flu," NSW chief health officer Dr Kerry Chant said.
"We don't want to see a repeat this year, so we are reminding families to book their children aged under five years in for their free flu vaccine now."
Head of the interim Australian Centre for Disease Control Professor Paul Kelly said the highest notification rate for flu in 2023 was in children aged under 14 years.
"But concerningly, the vaccine uptake was very low in this population group," he said.
Professor Kelly said last year's flu season was a "tragic reminder" influenza was not the same as the common cold.
"It is a serious virus that can cause severe illness, hospitalisation and death among otherwise healthy children and adults," he said.
"I encourage everyone 6 months of age or over to get vaccinated against the flu. It could save your life."
As well as children aged six months to five years other groups considered at a higher risk include those aged 65 and over, Aboriginal people, pregnant women and people with serious health conditions.
People in these priority groups are eligible for free flu vaccines while others can purchase the vaccine through GPs, pharmacies and immunisation clinics.
