Urgent genomic testing is underway for three people who arrived from southern Africa overnight to determine if they have the new Covid variant of concern, Omicron.
NSW Health said one of the three people, who arrived on Sunday night, was likely to be considered "an old infection".
Results of the genomic testing for the three newly arrived cases, who are isolating in special health accommodation, are expected on Monday evening.
This comes after two travellers from South Africa tested positive for Covid-19 after landing in Sydney on Saturday and later they were confirmed to have been infected with the variant of concern.
On Monday morning, NSW Premier Dominic Perrottet announced there may have been a third possible case of the Omicron variant.
But NSW Health says initial testing "indicates this person is unlikely to have been infected with the Omicron variant".
"Over the last 24 hours we've had 141 people from those nine affected countries come in, there are two, possibly three, positive cases," Mr Perrottet said on Monday.
In the 24 hours to 8pm on Sunday, NSW recorded 150 new infections, but six cases have been excluded after further investigation.
Australia has shut its borders to non-citizens coming from nine African countries – including South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini, Seychelles, Malawi, and Mozambique.
"We just don't need to live alongside Covid, we need to live alongside the variant as well," Mr Perrottet said.
"We're very confident based on our high vaccination rates... vaccination is key to NSW being able to continue opening up safely.
"In addition to that... our booster shot program is up and running as well.
"Those people across the state where it's been six months since your second-dose vaccination, please go out and get a booster shot."
Australia to reconsider reopening to visa holders
Australia's national security committee will revisit the country's scheduled reopening to students and visa holders in light of the new variant.
The committee will meet on Monday afternoon to look at whether Australia can reopen to double-dosed visa holders, skilled workers and international students from Wednesday as scheduled.
Federal, state and territory leaders are also expected to meet within the next 24 hours to consider Australia's response.
Prime Minister Scott Morrison has urged calm after the strain was confirmed in two fully vaccinated people travelling from southern Africa who arrived in Sydney over the weekend.
"It is important we just calmly and carefully consider this information, work together, take the decisions that are necessary and that is exactly what everybody is doing," he told the Seven Network on Monday.
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