Revelations that MPs' emails are being screened, and even blocked, has sparked an investigation at Parliament.

Labour made the discovery when MP Chris Hipkins tried to send information to a journalist on Monday night.

But it was blocked, because Parliamentary Service found it contained "sensitive words".

"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.

Mr Hipkins said they have no right to screen emails sent by Members of Parliament, and he's now realised how widespread it is.

"Some of our staff have actually been emailing me with other examples of where their emails have also been blocked.

"They're all recent examples, like within the last few weeks. So I'm not sure how far back this goes.

"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.

Speaker David Carter is responsible for it.

He said he would look into Mr Hipkins' complaint and report to Parliament on Wednesday.

Additional reporting by NZ Newswire