An embattled National Party backbencher's been cut off from his leader, while facing a police investigation.

John Key's refusing to say what he knows about the investigation into Northland MP Mike Sabin over an alleged assault - or when he first found out.

Mr Sabin is chair of Parliament's law and order select committee.

The PM was asked whether they spoke late last year.

"Uhh not other than at Caucus, no."

Police have refused to even confirm the investigation, which is understood to have been transferred to Auckland from Whangarei, where Mr Sabin served as a police office.

Key is uncertain whether Northland MP Mike Sabin will make an appearance at Te Tii Marae, alongside his party colleagues.

But the Prime Minister says he'd be surprised if the local MP didn't show up.

"I would be but just wait and see. It'll be my expectation that all members who have indicated to the whips that they're going would be there.

"I don't know if he has indicated to whips if he is going or not."

National's senior whip Tim Macindoe expects Mike Sabin to attend.