The fledgling political career of Northland MP Mike Sabin is under threat after a report that he's under police investigation over an alleged assault.

All those who may know details about the complaint, including police, and the Prime Minister's office, have refused to shed light on it.

However it's understood senior Northland National Party figures have already been discussing potential candidates to replace him in the event of a by-election.

Mike Sabin is a former detective who turned into an anti-drug campaigner who has held Northland for just over a term.

The investigation into events is being handled outside his old police district.