Education Minister Hekia Parata is denying claims by a union that her office accessed teachers' personal information to expose their political opinions to their employers.
The Post Primary Teachers Association says several teachers who wrote to Ms Parata about student/teacher ratios got copies of a response that was addressed to their school board and principal.
The union says the teachers sent letters from personal email addresses and did not say where they work.
PPTA president Robin Duff says he knows of three cases and this suggests Ms Parata's office accessed their teacher records or matched their names with publicly available information.
A spokesperson for the minister says there is no way she asked staff responding to the letters to look up the teachers' records or trawl the internet for clues about where they work.
However, the PPTA would not allow Radio New Zealand to see the full details of the letters, saying that would infringe on the privacy of the teachers who wrote them.