Prime Minister John Key's office is refusing to comment on reports of an assault complaint against National's Northland MP Mike Sabin, directing media queries this afternoon to police.

It was this morning reported that former Northland police officer Mr Sabin is under investigation over an assault complaint.

However it has also been reported the investigation is being handled by Waitemata police, rather than police in Northland, where it was claimed the events at the centre of the complaint took place.

Mr Key's office this afternoon refused to answer questions on the matter, directing the Herald to police.

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However police were similarly refusing to comment on the matter, saying they did not respond to requests seeking to confirm whether complaints had been made, or if investigations were under way against any individual.

Mr Sabin has so far not responded to requests for comment.

Mr Sabin, 46, has been National's MP for Northland since 2011 when he gave up his anti-methamphetamine consultancy business Methcon which he formed following his departure from the police in 2006.