This really puts National in a difficult position.
The next election is shaping up to be a law & order election. National's "tough on crime" narrative is going to be a whole lot harder to sell with this scandal hanging over them. On the other hand, if they ditch him then they will get brutally punished in the by-election that will follow.
The big winner here could be Act. They put forward a strong candidate in the Tauranga by-election and a lot of National's voters would support him if another by-election were to be held.