The Ministry of Education's new pay system has identified an error that has cost the Government $420 million over the past 10 years.
The system, introduced on Monday 20 August, spotted a long-standing problem in the way schools' short-term relief staffing is calculated.
The ministry says some schools got an inflated entitlement to short-term relief teachers and the cost of the error over about 10 years amounts to $420 million.
The Government has decided to let schools keep the money, but is working to ensure it is allocated evenly.