There are now 25 cases of the Omicron variant of COVID-19 in NSW, a jump of 10 in a day.

NSW Health said 11 of the Omicron cases were acquired either overseas or on an international flight.

None of the people infected with Omicron has been admitted to hospital in NSW for treatment.

Meanwhile, NSW authorities are trying to track the source of an Omicron cluster at St Peter Chanel Catholic Primary School in western Sydney and Sydney Indoor Climbing Gym in Villawood in south-west Sydney.

A student at the Regents Park school is believed to have acquired Omicron at the climbing gym, forcing all year three and year four students and teachers into isolation.

NSW Health said genome sequencing carried out on Sunday found an additional nine cases in the cluster carry the Omicron variant, bringing to 14 the number of cases linked to the school and the gym.

That number does not include a Canberra person who is believed to have contracted the virus at the climbing centre on November 27.

Meanwhile, there were 208 cases of COVID-19 in NSW and no deaths recorded from 61,132 tests in the 24 hours to 8pm on Sunday.

The state’s adult vaccination rate remains at 94.6 per cent for first-dose coverage while 92.8 per cent are fully vaccinated.

More than 81 per cent of teens aged 12 to15 have had one dose of the COVID vaccine and 77.2 per cent are fully jabbed.

There are 152 people with COVID-19 in NSW hospitals. They include 24 in intensive care and five on ventilators.

Chair in epidemiology at Deakin University Catherine Bennett said the strain was spreading faster than the Delta variant, but the symptoms were less severe.

“You do not want that increase in infectiousness coinciding with even the same level of illness,” Professor Bennett told ABC TV on Monday.

“The fact that it is presenting with more mild illness so far is really encouraging, given that high rate of spread.

“If it does just have that edge on Delta and were to replace Delta in other settings, then it’s important that it’s not putting more pressure on hospital systems.

“It would be even better if it was putting less [pressure on hospitals],” she said.

Meanwhile, a pilot program to return international students to the state starts on Monday. Up to 250 students are expected to arrive on a charter flight before isolating in student accommodation for three days.

Other international students will have to wait until December 15, after the federal government paused the easing of border restrictions due to the Omicron COVID-19 variant.