Students could bring COVID-19 onto school campuses and spread it to classmates before they are required to do a rapid antigen test, NSW authorities have conceded.

Responding to criticism that some schools are yet to receive RATs, Premier Dominic Perrottet said the twice-weekly testing regime didn’t necessarily have to begin on day one of the school term.

“The tests don’t need to be done prior to the first day of school,” he told reporters on Thursday.

“There was never a requirement they be there on the first day.”

Education Minister Sarah Mitchell had earlier asked “all of our students and staff to test before they come back to school”.

Six million RATs are being distributed to the state’s 3000 schools to enable students and teachers to test themselves twice a week throughout February.

Some private schools began classes on Thursday, while most state school students begin on Tuesday.

The NSW government has left finer details, such as specific days to test and whether to tally results, to individual principals.

Catherine Bennett, Deakin University’s Chair in Epidemiology, expects more COVID-19 cases will be found, particularly recent asymptomatic infections.

“Particularly with kids, they’re likely to be picked up in the first week of school screening,” she told 2GB.

Authorities reiterated the need to raise vaccination rates, with a small uptick in the 12-to-15 age group not enough to reduce the fact one in five of those children remained unvaccinated.

The third-dose rate among adults is also moving slowly — from 29 per cent to 36 per cent in the past week.

NSW Health Deputy Secretary Susan Pearce said about 100,000 vaccination bookings went begging at state-run clinics last week.

“There’s a perception in the community that Omicron is milder and so therefore the booster is not necessary,” she told reporters.

“What we know is that to prevent severe disease associated with COVID, that booster is absolutely critical.”

About seven per cent of NSW adults are either unvaccinated or have had just a single dose. They made up 31 per cent of the deaths reported on Thursday.

Ten women and 19 men died, including two people in their 60s and 19 who were 80 or older.

The state recorded 17,316 cases in the 24 hours to 8 pm on Wednesday, shifting the seven-day average below 20,000 cases for the first time since January 4.

Hospitals are treating 2722 COVID patients, down 72, while the number of intensive care COVID patients rose to 181. Of those, 72 are on ventilators.

– AAP