Classroom repairs and a ballooning student population will cost the Government $1.1 billion more than expected.
"When combined with the need to fund urgent repair work and there being no money in the kitty to finish the Christchurch school rebuild programme, we're faced with a whopping $1.1 billion bill over the next four years," Education Minister Chris Hipkins said.
It is the latest in the rollout of areas Labour claims National left a "hole" it had not budgeted for as it prepares to unveil its first Budget.
But National claims Labour has not allowed for the usual cost pressures in its pre-election Budget plan.
Prime Minister Jacinda Ardern has said health and education are the primary areas where Labour needed to play catch-up, but it went across the public sector.
Labour also warned would need to compromise on some of its own policy areas. It has already warned it will not be able to deliver on its promise of cheaper GPs visits by the expected date of the middle of this year.
Hipkins said the previous Government had turned a blind eye to the impact of a growing population and left the new Government to pay for schools and classrooms for 17,000 more students than expected.
"National pursued population growth at all costs, but did not plan for the costs. It's one thing to try to boost the economy by increasing the population size, but not planning to pay for the infrastructure to support those extra people is short-sighted."
The previous Government approved $1.137 billion for the Canterbury school rebuild programme but had not set aside the full amount, he said.
"That has left a significant portion of it still to be paid. The people of Christchurch, however, can be assured this Government will make good on the shortfall and the building programme will continue unaffected."
National's education spokeswoman Nikki Kaye said the Government was crying poverty after failing to budget for business-as-usual spending.
"Chris Hipkins says he has identified a 'surprise' $1.1 billion of school property projects that need funding. This is embarrassing for the Minister, because the projects are just part of business-as-usual that the Government needs to fund each and every Budget.
"Over the last three Budgets, for example, National set aside an extra $1.7 billion for school property – taking the total funding available for school infrastructure over the next four years to $4.8 billion.
Investing in school property to get ahead of growth was a priority for National and was part of the business-as-usual spend of any government.
"We were on track to deliver the 17,000 extra student places needed in Auckland by 2019, and were finalising the Auckland Education Growth Plan to strategically manage the increasing pressure on Auckland's schooling network.
"We knew that growth was continuing and would need further funding each year," Kaye said.