A Year 11 student is the latest confirmed case of COVID-19 in NSW, as the number of cases hits 25.

The number of NSW coronavirus cases has more than doubled in a week after a Sydney high school was shut down after a Year 11 student tested positive.

Almost 1200 pupils at Epping Boys High School have been affected by a state government decision to impose a 14-day quarantine period, meaning students and school staff should stay at home.

"Our first concern is for the health of the school's students and it was decided that until we learned more about this specific case, the school should be closed as a precaution," Premier Gladys Berejiklian said in a statement.

NSW Health Minister Brad Hazzard on Friday implored parents to not panic and said the ill student was "not bad, but he is not well".

"We expect over the weekend there will be a detailed examination of where the student has been since he contracted the virus and with whom he has been in contact," Mr Hazzard said in a statement.

Some 25 people have been diagnosed with COVID-19 so far, including a Goulburn man who returned from Singapore and travelled to Darwin. Nine of the 25 cases were a result of local person-to-person transmission.

"So it's almost tripled just in a week," Mr Hazzard told the Nine Network.

Meanwhile, 17 children are being tested after they visited a Sydney aged care facility where an elderly female resident, 95, died earlier this week and was later found to have been infected with the virus.

The children from Banksia Cottage child care centre visited residents at the nearby Dorothy Henderson Lodge at Macquarie Park on February 24.

Two other residents, aged 82 and 70, have also been infected after an aged care nurse working at the facility tested positive for COVID-19.

NSW Chief Medical Officer Dr Kerry Chant has warned more residents at the facility could be diagnosed with the virus.

Dr Chant also told aged care facilities to stop hosting children from child care centres to reduce the risk of infection for Australia's most elderly and frail.

Elsewhere, Mr Hazzard said two Sydney doctors who have the virus attended the same medical workshop on February 18, alongside about 70 other medical professionals.

The doctors, from Ryde and Liverpool hospitals, are so far the only ones who are sick from that group, and 14 days have passed, giving authorities hope that no one else has been infected.