Labour MP Chris Hipkins says the Parliamentary Service is monitoring MPs' emails, and he's asked Speaker David Carter to investigate.
Mr Hipkins told Parliament on Tuesday he had wanted to send an email to a member of the press gallery on Monday night.
"It was blocked and I was not able to send it because the document that I was attaching, that I had obtained under the Official Information Act, previously had a government security classification," he said.
"This is a very serious matter because it doesn't matter how I came to hold the document, the fact that the Parliamentary Service are monitoring the emails coming to and from MPs' email addresses is completely unacceptable."
The Parliamentary Service manages parliament and its communications systems.
Mr Carter is responsible for it.
He said he would look into Mr Hipkins' complaint and report to Parliament on Wednesday.