The NSW government is preparing for coronavirus to reach a "concerning phase" over the coming months.

Premier Gladys Berejiklian has urged people to keep things in perspective and to stay home if they become unwell while insisting the health system is on high alert as the number of COVID-19 cases rises.

"We are anticipating this virus will have a concerning phase of a number of months ahead of us," Ms Berejiklian said.

It comes as authorities work to determine how a 16-year-old Sydney student contracted the virus.

READ MORE Second aged-care nurse tests positive for coronavirus as nearby high school closed

Epping Boys' High School was closed on Friday as a precautionary measure after the boy tested positive for COVID-19.

The student is the son of a Ryde Hospital healthcare worker who had contact with the 53-year-old male doctor who has coronavirus.

NSW chief health officer Dr Kerry Chant said the government is "prepared to take whatever action is in the best interests of the students and staff and the broader community" and could close the school for up to two weeks.

And the NSW education department is preparing for the likelihood of more schools shutting down by ensuring students will be able to access classes online, The Daily Telegraph reported.

READ MORE Aged care resident confirmed as Australia's second coronavirus related fatality

Meanwhile, a 21-year-old man and a 24-year-old woman who worked at the Dorothy Henderson Lodge nursing home in Macquarie Park have also been diagnosed with COVID-19.

An 18-year-old woman on Friday became the state's 28th case after she had been in close contact with a previously known case in western Sydney.