The Post Primary Teachers' Association says an overpayment of $2.5 million to private schools by the Ministry of Education is a double blow.
The ministry says the 2011 funding rates for private schools were set incorrectly, resulting in the overpayment.
When giving an extra $10 million allocated to private schools in last year's Budget, the ministry failed to exclude money meant for new scholarships to help low-income families pay private school fees.
The ministry says it won't be ask schools for the money back and will find the funds from within existing budgets.
But the PPTA says that money will have to come from state sector provisions.
President Robin Duff says it is a double whammy as the private schools will get extra funds, but at the expense of state education.
Mr Duff says the schools involved should make arrangements with the Ministry, to repay the money that was overpaid.