Chief Health Officer Kerry Chant said there were more than 1024 cases reported last week, up from 478 the previous week.
"Alongside the increase in flu cases, we are seeing more people with flu presenting to and being admitted to hospital, including among young children," Dr Chant said on Thursday.
Authorities have been predicting a particularly bad flu season after two years of the virus being suppressed by restrictions on international travel and social distancing measures to combat the COVID-19 pandemic.
No deaths were recorded from the flu last year but vaccination coverage also dropped, leaving people vulnerable to new influenza strains.
In 2019 - the last winter before the pandemic - more than 313,000 flu cases were reported in Australia and 902 people died.
NSW Health's respiratory surveillance report for the week ending April 30 indicated this year's winter flu season was likely to start early.
Influenza cases were most noticeably on the rise in people aged in their 20s (333 cases) and those living in south eastern Sydney (253 cases).
More people, and specifically more children, are presenting at emergency rooms with respiratory problems, fever and unspecified infections and more of them are being admitted than they were in 2017-2019.
The number of people being diagnosed with COVID-19 was decreasing or stable across all ages and health districts.
But the number of PCR tests being conducted to detect the virus had dropped while the proportion of tests that returned positive results remained stable at 17 per cent.
Health Minister Brad Hazzard acknowledged there was vaccination fatigue in the community but urged anyone over the age of six months to get a flu shot as soon as possible.
"It is crucial everyone gets vaccinated against flu to not only protect themselves, but their colleagues and loved ones against serious illness or worse," he said.
People over 65, children under five, those who have serious health conditions, are pregnant, Aboriginal or Torres Strait Islander are at higher risk of severe illness and can receive a free flu vaccination.
Along with flu shots, NSW Health is also encouraging people yet to receive a booster shot of a COVID-19 vaccine to get one, noting it is safe to get both jabs at the same time.
Opposition health spokesman Ryan Park said flu season would be a very challenging period and people needed to be pushed to get vaccinated against influenza and COVID-19.
"We've still got a lot of staff furloughed, we've still got a lot of staff experiencing a very challenging set of circumstances," he said.
"We know a bad flu season can have huge impacts on public hospitals and the health system."
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