NSW Premier Dominic Perrottet says there are "possibly" three cases of the Omicron variant in returned travellers when it was previously believed to be two.
Mr Perrottet revealed the third possible case during a press conference on Monday after Australia closed its borders to nine southern African countries over Omicron concerns on the weekend.
"Over the last 24 hours, we had 141 people from those nine affected countries come in. There are two, possibly three, positive cases," Mr Perrottet said.
"Those 141 travellers on five flights over the last 24 hours are in our health quarantine facility."
Although he has ordered all international arrivals to quarantine at home for 72 hours, Mr Perrottet insisted the NSW international and state borders would remain open.
"Ultimately we need to open up to the world (and) we need to do so safely," he told reporters on Monday.
"We don't need to have a knee-jerk reaction, we need to have a proportionate and balanced response to the situation that's in front of us."
The three-day quarantine order is on top of a federal government requirement for travellers to enter quarantine for two weeks if they have been in South Africa, Lesotho, Botswana, Zimbabwe, Mozambique, Namibia, Eswatini, Malawi or the Seychelles in the past 14 days.
Mr Perrottet also stressed NSW was better equipped to confront new variants.
"We've got to learn to live alongside the variants of the virus that come our way," he said.
"And the vaccination rate here is one of the highest in the world.
"That is not the case in the southern African nations."
The Premier has said there are no plans to adjust the state's reopening roadmap, so restrictions will still ease for the unvaccinated on December 15.
Meanwhile, the state added 150 new infections to its caseload on Monday.
For the fifth consecutive day, no new deaths were reported.
Hospitals are treating 170 patients, five more than the previous day, including 25 people in intensive care.
NSW is 94.5 per cent single-dosed for everyone 16 and over, while 92.4 per cent are fully vaccinated.
Of 12- to 15-year-olds, 81.3 per cent have received one jab and 76.5 per cent both.
- With Australian Associated Press