Dozens of people are in isolation, a school has been closed and infections are rising in Sydney's northern suburbs, which have emerged as the epicentre of the city's coronavirus outbreak.

Key points: A second staff member at Dorothy Henderson Lodge has tested positive to COVID-19

A second staff member at Dorothy Henderson Lodge has tested positive to COVID-19 Epping Boys High School was also closed after a student tested positive

Epping Boys High School was also closed after a student tested positive NSW Premier Gladys Berejiklian has called for calm amid rising infections

There are now 28 confirmed COVID-19 cases in NSW.

The latest case to be diagnosed is an 18-year-old female in Western Sydney.

Two staff members from Dorothy Henderson Lodge aged care facility in Sydney's north tested positive today.

A third worker at the nursing home tested positive earlier in the week.

Four residents at the lodge have contracted the virus, and one of them, a 95-year-old woman, died earlier this week.

Yesterday parents and children were tested after a group of 17 children from the Banksia Cottage, at Macquarie University campus, visited the nearby nursing home on February 24.

None of those tested positive for coronavirus.

Coronavirus infections in northern Sydney.

Nearby, Epping Boys High School was closed today after a 16-year-old student was diagnosed with coronavirus.

His mother works at Ryde Hospital, where a 53-year-old male doctor was diagnosed with coronavirus.

The school, hospital, aged-care facility and child care centre are all within about 4km of each other.

A total of 117 people have been placed in isolation from the hospital, including 56 patients, while staff and students at the school have been advised to self-isolate this weekend.

A statement from the school read: "To provide time for NSW Health to conduct their contact tracing process the school will be non-operational Friday 6 March, 2020."

It is the first time a case of COVID-19 has been found inside a school in Australia.

NSW Premier Gladys Berejiklian said the decision to close the school was precautionary, and urged people to remain calm.

"It's given all of us [and] the experts a breather to assess the situation over the weekend to make a decision on Monday," she said.

She also said the Government was "doing everything we can to reduce the spread of the virus".

"We are going to see cases escalate and we are going to see challenges in terms of decisions we take around how to contain and reduce the spread," she said.

Deputy secretary of education Murat Dizdar said investigations into how the boy contracted the virus were continuing.

"We're asking that students and teachers isolate over the weekend as we gather more information with further advice that we'll provide to the school community," he said.

NSW Premier Gladys Berejiklian has called for calm while Health Minister Brad Hazzard calls the school closure "sensible". ( AAP: Joel Carrett / Julian Andrews )

The NSW Health Minister Brad Hazzard compared efforts to track down those who may have been infected to a "police investigation".

"We want to do what we can for containment but it's getting much harder," he said.

He said closing down the school was "a sensible outcome".

"We're lucky it's so close to the weekend," he said.

"A good decision has been taken, and some kids will be quite happy they've got the day off."

The Department of Education said it will implement a cleaning program for the school after advice from NSW Health.

While the department has been advised that the risk is low, the cleaning is being conducted to provide additional confidence to students and staff of the school.

A health care worker at Canterbury Hospital tested positive to COVID-19 on Wednesday, having recently returned from Iran.

She worked while she was infectious between February 25 and March 3.

Staff members who were identified as close and casual contacts are all in home quarantine.

A further three contacts are being tested, and a remaining staff member was being followed up today.