Authorities have no idea how the high school student in Sydney caught the virus, with no recent overseas travel history or connection to anyone who has recently been abroad.
The case is a student at Regents Park Christian School in western Sydney.
The senior school has been dismissed for the year.
There are nine cases of the Omicron variant in New South Wales.
There are currently 140 people in hospital with the disease, 25 of which are in intensive care. Ten of those require ventilation.
In the state, 94.6 per cent of people aged 16 and over have received a first dose of a COVID-19 vaccine, and 92.6 per cent are fully vaccinated.
In children aged 12 to 15, 81.3 per cent have received a first dose of COVID-19 vaccine, and 76.9 per cent are fully vaccinated.
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