The fully vaccinated traveller had been in southern Africa and was in hotel quarantine since arriving on Sunday's flight SQ231, it was confirmed late on Thursday.

Every person on the flight should already be in quarantine, or a 72-hour isolation period while waiting for a negative test result.

The passengers will be due to get tested again on Saturday, six days after entering Australia.

Earlier on Thursday, a child was confirmed as the seventh returned traveller to be diagnosed with the Omicron variant. Health authorities fear they may have contracted the virus on their flight, given they had not been in any of the southern African countries of concern.

The child, who was too young to have been vaccinated, arrived on flight QR908 from Doha to Sydney on November 23.

Two members of their household also have COVID-19 and urgent genomic sequencing is underway to confirm whether they have been infected with the Omicron strain.

The traveller is isolating in Special Health Accommodation with their family, NSW Health said on Thursday.

"These travellers have not been in southern Africa and NSW Health is concerned transmission may have occurred on this flight," the authority said.

Everyone who arrived on the same flight is being asked to get a PCR test and isolate while they wait for further advice.

The traveller stayed at the Mantra Serviced Apartments at Chatswood in Sydney's north for seven days from the day they arrived.

Anyone who visited the apartments during that week is considered a casual contact and is required to get tested and isolate until they receive a negative result.

Health Minister Brad Hazzard is urging everyone who is eligible to get a booster shot to maintain protection, after a 40 per cent increase in the uptake since last week.

Mr Hazzard said 500 previously tested swabs of travellers who had returned to NSW since November 22 had been re-tested to ensure they did not contain the Omicron strain.

The jury is still out on whether the latest strain is more dangerous and could overwhelm hospitals, or merely more transmissible and able to replace stronger variants.

At least two people with the Omicron variant have been in the community because they landed in Australia before quarantine rules changed on Saturday.

A woman in her 30s from the Central Coast and a man in his 40s from Cabramatta in Sydney's southwest are both infected with Omicron and arrived on the same Qatar Airways flight from Doha a week ago.

Mr Hazzard said the Cabramatta community was on high alert but stressed the infected man had only visited one shop on Monday.

"There have been no further indications of transmission in that area," he said.

There were 271 cases recorded in NSW in the 24 hours until 8pm on Wednesday from 81,877 tests, and no deaths.

It is less than two weeks before more restrictions are lifted in NSW, when the unvaccinated will be able to return to hospitality venues and all shops.

Some 94.6 per cent of people 16 and older have had one vaccine dose while 92.6 per cent of adults have had both.

More than 81 per cent of teens aged 12-15 have had one shot while 76.8 per cent are fully vaccinated.

There are 144 people in hospital, including 24 in ICU and 10 ventilated.

Australian Associated Press