NSW has reported 149 new coronavirus cases in 24 hours, the highest one-day jump since the outbreak began, as confusion over whether children should be going to school continued.

The new cases bring the state's total number of confirmed cases to 818, NSW health authorities said on Tuesday morning. Australia's most populated state has been the hardest hit by the pandemic, recording seven of the country's eight COVID-19 related deaths.

As of Tuesday morning, 13 people were in the ICU with serious cases of the virus in NSW.

Premier Gladys Berejiklian described the increase as "significant" and warned that the state had reached a "critical" stage in the outbreak.

"We have to maintain the spread, control the spread, and this is exactly the critical time in NSW for that to occur," she said. "I don't want to be another example of a jurisdiction that didn't do what it needed to do at the right time."

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Ms Berejiklian urged people to follow social distancing rules or face "harsh penalties" as the state ramps up compliance measures.

"If they're supposed to be in self-isolation and they're not, there are harsh penalties and we'll enforce that. We have to take this seriously," she said.

NSW Chief Medical Officer Kerry Chant said the majority of cases were a result of people returning to Australia from overseas and stressed the importance of complying with the 14-day isolation requirement.

"We are seeing an incredible burden in our recently returned overseas travellers and it is essential to protect the broader community and those that you love to maintain that social isolation," Dr Chant said.

Ms Berejiklian also used the media conference as an opportunity to hit back at critics who have accused the government of mixed-messaging regarding whether children should go to school.

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The Premier has recommended that parents keep their children home unless they are an essential service worker, but maintained that schools across the state would remain open.

"[There are] essential workers, our front-line staff, those who are keeping the community operating during this time of uncertainty," she said.

"But we also know that many, many parents are already choosing to keep their children at home."

NSW Education Minister Sarah Mitchell said on Tuesday that the school absentee rate across the state skyrocketed to 41 per cent on Monday, up from an average rate of 8 per cent.

The state's peak body representing teachers has slammed the Federal Government's "hypocritical and contradictory" position that schools should remain open despite banning other large gatherings.

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In a statement on Monday, the NSW Teacher's Federation called for greater health protection for teachers - who they said have been "thrust onto the frontline of the crisis" - and increased support for a "core" group of staff to provide supervision for children of essential workers.

The comments follow the decision of Prime Minister Scott Morrison to keep schools open following a national cabinet meeting on Sunday, but he added that parents who wished to keep their children home would be permitted to do so until "the end of this year's school term".

Hours before the meeting, Victorian Premier Daniel Andrews announced that the school holidays would be brought forward in the state, effectively closing schools from Tuesday.

As of Tuesday afternoon, 1,895 cases have been recorded across Australia as Queensland, Tasmania, South Australia and the Northern Territory close their borders to domestic travellers.

Anyone entering the states and territories from other parts of Australia will be subject to a 14-day isolation period in line with the requirement for people returning from overseas.

Coronavirus symptoms can range from mild illness to pneumonia, according to the Federal Government's website, and can include a fever, coughing, sore throat, fatigue and shortness of breath.

Only people who have recently travelled from overseas or have been in contact with a confirmed COVID-19 case and experienced symptoms within 14 days are currently advised to be tested.

But on Monday night, Australia's deputy chief medical officer told ABC's Q+A that the guidelines for COVID-19 testing were likely to be expanded this week due to the strict limits on overseas travel.

If you believe you may have contracted the virus, call your doctor, don’t visit, or contact the national Coronavirus Health Information Hotline on 1800 020 080.

If you are struggling to breathe or experiencing a medical emergency, call 000.