An error with school payroll system Novopay involved the private details of 530 staff at 10 schools, the company behind it says.
Principals today said they had been sent the personal details and pay rates of principals and teachers at other schools.
They said they had been asked by Novopay to destroy downloaded copies of the reports.
Crown-owned Education Payroll Limited, which runs the payroll system, said the information was accessed by 552 "authorised users" - either principals or school administrators.
It said the users would have been accessing the system in order to check that next week's pay-day would be correct.
But, instead, each of them would have been able to see the data from one of the 10 affected schools.
Its COO, Duncan Boennic, said the lapse was deeply regrettable and it apologised for it.
"The error has arisen as the result of a recent software update. We will fully investigate how the error occurred," he said in a statement.
The Office of the Privacy Commissioner had been notified, the company said.
'Significant privacy breach'
Teachers' union NZEI earlier said it was a "significant privacy breach".
NZEI national secretary Paul Goulter said even one mistake was too many when it came to people's privacy.
"This is yet another Novopay shambles that puts individuals' privacy at risk and puts more strain on school staff at a stressful time of year," he said.
The payroll system was dogged by problems when it was introduced in 2012 and cost millions of dollars to fix.
Minister of Education Hekia Parata said fixing the breach was a priority, and the company in charge of the system was committed to solving the problem.
"Of course we're concerned about it," she said.
"Let's be clear, this has been significantly improved since 2012 and Payroll Limited has been doing a very comprehensive job. So they will want to know how this breach occurred so they can address it."
The Novopay website said schools would not be able to download their payroll information until tomorrow.