National's Northland MP Mike Sabin's three-year political career is hanging in the balance after he was yesterday reported to be under investigation by police following an assault complaint.

Prime Minister John Key refused to comment on the report yesterday, instead directing media queries to police. However, police were also refusing to confirm either receiving a complaint or any investigation into allegations against the former detective policeman and anti-methamphetamine campaigner.

Nevertheless, the Herald understands senior Northland National Party figures have already been discussing potential candidates to replace him in the event of a by-election.

National Party president Peter Goodfellow said party hierarchy were unaware of any allegations, either historical or more recent, against Mr Sabin, 46, ahead of this year's election.

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The investigation into the assault complaint against Mr Sabin is being handled by police in Auckland's Waitemata district rather than in Northland where Mr Sabin served as a detective and where it was claimed the events at the centre of the complaint took place, it was reported yesterday.

Mr Sabin, who lives in Coopers Beach with his partner, did not respond to calls from the Northern Advocate.

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