The Prime Minister is having to explain why his party's doing so badly in a seat that should have been safe.

Mr Key's come straight home from Japan and spent today campaigning in Northland.

Two polls now show his candidate Mark Osborne is well behind, and set to lose to New Zealand First leader Winston Peters.

John Key says by-elections are always quirky and they often end up in this situation.

"And you have got someone who's got amazing name recognition with Winston Peters, up against a new candidate. That's always going to be the case with Mark, someone coming into Parliament is always going to be new."