New Zealand First MP Ron Mark has apologised after he was caught swearing on a live microphone in Parliament's debating chamber.

Mr Mark was heard saying "Shut the f*** up" under his breath during an exchange with the Speaker David Carter this afternoon, after he had been repeatedly interrupted.

The comment was barely noticed at the time, but Leader of the House Gerry Brownlee read tweets about it and raised it in the House.

The swearing was also picked up by Parliament TV's sign language interpreter, who happened to be appearing as part of Sign Language Awareness Week.

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Mr Mark apologised to MPs, saying the comments "could have brought the House into disrepute".

Leaving the House, Mr Mark at first claimed he had only mouthed the words. When told it had been picked up by microphones, he admitted it was out of order.

The MP said he had been referring to a "group of MPs that were behaving the way they were".

Mr Brownlee said the swearing was inappropriate but Mr Mark had now apologised.

New Zealand First leader Winston Peters would not comment because he did not hear the original remark.

Asked whether he had "dropped the f-bomb" in the House, he said: "Well ... look, not so that you heard it."

It is not the first time Mr Mark has been caught out. He was filmed giving the fingers to National MP Tau Henare across the debating chamber in 2006.

Former Labour MP Steve Maharey was also forced to apologise in 2007 when said "f*** you" to National MP Jonathan Coleman in the House during an argument.