NSW Health is urgently contact tracing 140 people who signed in with a QR code before boarding the Cadman Cruise at King Street Wharf on Friday night, after five of them contracted the virus.

Chief Health Officer Kerry Chant says preliminary tests show two of the five cruisers probably have the new Omicron variant.

Anyone who was on board, and everyone in their households, must immediately get tested and isolate.

There are now 34 cases of Omicron in the state - 21 of them linked to a cluster that started at a climbing gym in southwest Sydney and 13 who are returned travellers.

"We want to suppress the spread of Omicron as much as we can as we learn more about it," Dr Chant said on Wednesday.

NSW Health was using QR codes "to keep those numbers low, particularly as we move into that Christmas-New Year period", she said.

There were 403 new COVID cases in NSW in the 24 hours until 8pm on Tuesday, a jump of 143 infections on the previous day.

Dr Chant said the spike in cases was due to increased transmission in large venues such as pubs, clubs and parties as people began socialising freely again.

Transmission was "predominantly occurring in vaccinated individuals" who experienced mild symptoms, she said.

"So my key message is don't attend any functions if you have any symptoms, however mild."

As a precaution she advised people without symptoms to undergo a rapid antigen test before attending anywhere where the would be "lots of people crowded, singing (and) dancing".

However, anyone with symptoms should always have a COVID test performed by a medical professional.

She also appealed to people to consider frail and elderly relatives at Christmas gatherings by being COVID safe and choosing outdoor venues if possible.

She stressed it was also important for people to get booster shots as soon as possible, particularly people with chronic health conditions and suppressed immunity.

"Talk to your GP because you may actually need a third dose as part of your primary course and that vaccine given at a reduced interval," she said.

The latest COVID death was a fully vaccinated man in his 80s with underlying health conditions from southwest Sydney.

The new cases were diagnosed from 92,566 tests, up from 58,706 tests the previous day.

The first dose adult vaccination rate in NSW is unchanged at 94.7 per cent, while 92.9 per cent of people 16 and older have had both.

More than 81 per cent of teens aged 12 to 15 have had one vaccine dose and 77.3 per cent are fully jabbed.

There are 151 people in hospital, with 25 in ICU and seven on ventilation.

Australian Associated Press