NSW Teachers Federation president Angelo Gavrielatos says the strike on Wednesday will go ahead as state school teachers seek pay rises of five to 7.5 per cent while struggling with their workload.
Principals have been told to advise their school communities of what impact the strikes could have, with most schools providing minimal supervision.
Mr Gavrielatos called on parents to support the industrial action to ensure their children are taught by qualified teachers, warning of an unsustainable situation in education with more than 70 per cent are considering new careers.
Education Minister Sarah Mitchell on Tuesday urged the union to cancel the strike after directing her department to delay wage negotiations as part of a new teachers' award until after the June 21 budget.
Ms Mitchell said delaying wage arbitration while continuing other negotiations showed the government was working in good faith.
Premier Dominic Perrottet said the government had been fair and reasonable throughout negotiations while public sector unions representing teachers, nurses, paramedics and rail workers had taken industrial action.
Describing the scheduled strike as "disappointing and ... immature", the premier defended his government's track record amid soaring inflation.
"Over the last 10 years... we've had public sector wages grow at 5.4 per cent ... Our wages in NSW for the public service have grown above inflation and above private sector," he told 2GB Radio on Tuesday.
The action was part of a political campaign by the unions to cause havoc across NSW for the benefit of the Labor Party, he said.
"They are completely politically motivated and it's not just the Teachers Federation... It's certainly aligned with the Labor Party."
"I can't guarantee the teachers will be happy where we land," the premier noted.
Opposition education spokeswoman Prue Car said the rising cost of living was the real factor driving calls for public sector wage increases, and the premier could prevent further strikes by revealing what would change after the budget.
Mr Perrottet promised to address public sector wages in the budget but also acknowledged that may not be enough to appease frontline workers.
Mr Gavrielatos said there was no guarantee the government would make good on its promise.
The union had been seeking negotiations since February last year and won't wait until it sees what's in the budget before striking.
"We have been very patient," Mr Gavrielatos said.
He dismissed temporary changes to assessment and accreditation requirements that could ease teacher workloads announced on Tuesday as "gimmickry".
Ms Car said the government needed to do more if it wanted to stop the strike.
"A last minute delay to the IRC negotiations is not going to cut it," Ms Car said.
"If Dominic Perrottet is going to find a way through this crisis with our teachers in the budget, why won't he announce it now so that this (strike) can be avoided."
The government had been warned about chronic shortages in classrooms over the last decade but not acted to fix them, Ms Car said.
Mr Gavrielatos said pay rises were needed to attract more people to the profession and to retain existing staff.
He accused the government of trying to address shortages by lowering qualifications and standards for teachers under the guise of modernising the profession.
Mr Gavrielatos declined to predict how many teachers would strike but expects a high turnout amid unprecedented anger across the profession.
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