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.