Two people who attended a Sydney boat party have tested positive for the COVID-19 Omicron variant, sparking concerns it could be a "transmission event".
Chief Health Officer Dr Kerry Chant said NSW Health is "urgently contacting" around 140 people who attended the party boat cruise on Sydney Harbour on the evening of Friday, December 3.
"Five people from this cruise have tested positive to COVID and we have preliminary testing, the special test that looks for the S-gene dropout, on two of these people and they have the Omicron variant," Dr Chant said.
"So we are concerned, we haven't got that result yet on those other three, but we are concerned this could be a transmission event associated with the Omicron variant. We want to suppress the spread of Omicron as much as we can as we move to learn more about it.
"Passengers and members of their households are advised to immediately get tested, isolate and await public health advice."
NSW COVID-19 cases spiked to 403 cases on Wednesday.
There are 34 cases of Omicron in the state but no one with the new strain has been admitted to hospital for treatment, NSW Health said on Wednesday.
The new case tally is a jump of 143 infections on the previous day's numbers.
A fully-vaccinated man in his 80s with underlying health conditions from southwestern Sydney died at Bankstown-Lidcombe Hospital in the 24 hours until 8pm on Tuesday.
NSW Health said preliminary results indicate two of the five COVID-positive party boat cruisers have the Omicron variant.
All are isolating at home.
Authorities are contacting about 140 people who signed in with a QR code before boarding the boat on Friday, which left King Street Wharf 9 at 7.30pm and returned about 11pm.
Anyone who was on board, and everyone in their households, must immediately get tested and isolate.
Operated by Cadman Cruises, the cruise was marketed as 'Flow Fridays - 90s Themed Boat Party' and 'Freaky Sunday Afrovibe: The Last Dance'.
Meanwhile, the Omicron cluster linked to a Villawood climbing gym in southwest Sydney and two schools has grown to 21, after genome sequencing identified another case.
Two more Omicron cases have been identified among recently arrived international travellers.
The number of Omicron cases in NSW acquired either overseas or on an international flight is 13.
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.
Chief Health Officer Kerry Chant on Tuesday said NSW Health was consulting with overseas counterparts to determine how contagious Omnicron is and how it is transmitted compared with other variants like Delta.
"We are also working to understand the severity of the new variant and importantly how effective vaccines are at offering protection against severe disease," she said.
- With Australian Associated Press