NSW Health says about 10,000 positive rapid antigen tests registered between Sunday and Monday are included in figures released on Wednesday due to a data processing problem.
"These positive RATs from Sunday and Monday artificially inflate the cases being reported today for the 24 hours to 4pm Tuesday," NSW Health said.
It comes a day after NSW reported 10,689 cases of the virus, up by more than 1700 from the previous day.
Five people died from the virus on Tuesday and 1016 people are in hospital, 36 of them in intensive care.
Health authorities are warning NSW to brace for a new wave of COVID-19, with cases likely to double by the middle of next month.
Health Minister Brad Hazzard says the same researchers who correctly predicted the Omicron summer surge have forecast another wave of cases that will peak next month.
Acting Chief Health Officer Marianne Gale says within weeks the new Omicron sub-variant BA2 will be "by far the dominant strain in NSW".
Former Australian Medical Association boss Tony Bartone is also warning about a winter surge of cases, saying BA2 and the cold season posed a new threat as people spent more time indoors together.
"The BA2 'son of Omicron' variant seems to be much more easily spread in family situations. Especially when children are largely unvaccinated," he told the Nine Network.
He urged parents to get their children vaccinated and eligible Australians to get a booster shot.
"We saw in January how quickly things can change," Mr Bartone said.
"Be wary, be vigilant. Don't forget the lessons of the past."