States and territories will go their own way on a safe return to schools despite weeks of leaders discussing a co-ordinated approach.
Prime Minister Scott Morrison said the commonwealth government will support states and territories that want to implement surveillance testing for schools on a 50-50 cost sharing basis.
But Mr Morrison stressed the health advice didn't recommend such a regime.
"While it is not the medical advice for (surveillance testing) to be undertaken, states and territories may be choosing to do that based on the education advice they are receiving to ensure schools can open and stay open," he told reporters in Canberra on Thursday.
"Where they believe that is necessary, the commonwealth government will be supporting that."
Mr Morrison said the Commonwealth would support the states despite it not being federal health advice because keeping schools open was a top priority.
"States and territories will make decisions to keep schools open. We need the schools open, we need them to stay open," he said.
"That is why we have agreed to be supportive and facilitative of that decision, where they choose to make it."
The definition of a close contact - at least four hours of contact with a COVID-positive household member - won't be extended to cover contact within schools grounds or buildings.
"If you are sick you should not be coming to school," Chief Medical Officer Paul Kelly said.
But Professor Kelly acknowledged that while the illness in children was mild, there was a risk a student would become infected and bring the virus home.
"To keep them at school as much as possible, there are trade-offs today in terms of transmission," he said.
"We want kids back at school and want them back on day one."
But Mr Morrison said previous data revealed the most likely source of infection for a child was at home and with Omicron rampant in the community, there is risk everywhere.
Prof Kelly said there needed to be minimal disruption to face-to-face learning but didn't directly answer a question on what isolation requirements would be in place if a person in a classroom tests positive.
Queensland and South Australia will delay the opening of school for the year in order to lift vaccination rates.
Labor frontbencher Kristina Keneally said there was no doubt schools needed some form of ongoing and regular rapid testing regime.
"Quite depressingly, Omicron may not be the last variant and we're going to need some type of system," she told Sky News.
"But we don't have it right now, we simply don't. While (surveillance testing) might seem good in theory or on paper ... the prime minister didn't order enough tests."
Finance Minister Simon Birmingham tried to allay fears there wouldn't be enough tests given their widespread shortage.
"Hundreds of millions extra (are) on order," he told the Ten Network.
"We're working with the states and territories to make sure we can provide what they need for the appropriate restarting of school and the support of testing where it's necessary in schools."
Speaking while national cabinet was meeting, Senator Keneally said there was no clear plan.
"We are days away from school, we have empty supermarket shelves, we have parents and teachers not sure what the rules will be for the safe return to schools," she said.
Mr Morrison said states and territories would all be following the national principles set out earlier in January.
The six principles included schools being the first places to open and last to close in an outbreak, baseline public health measures continuing, and responses being proportionate and based on health risk.
The prime minister also said the return to schools would not increase furlough rates during an outbreak, with modelling suggesting a return to school would reduce the number of workers unable to work at any given time by five per cent.
Australian Associated Press