National Party leader Simon Bridges says Labour's proposed repeal of the three strikes law will test the resolve of Winston Peters when he steps in as acting prime minister.
The 'three strikes' law is set to be axed, with Justice Minister Andrew Little saying he will take a proposal to Cabinet to repeal it.
It set up a three-stage system of increasing consequences for repeat serious violent offenders.
On a third 'strike', the sentence must be the maximum possible unless the court considers it would be manifestly unjust.
A number of victims of violent crime took out a full page advertisement in today's Herald. The open letter, backed by the Sensible Sentencing Trust, said "victims say no."
"We, the undersigned, are all personally the victims of serious violent crime, or close relatives of someone killed, maimed or sexually abused."
"We are all deeply concerned about the proposals to repeal the three strikes law, make bail easier, and generally to let people out of prison who need to be there," the letter read.
Mr Bridges said the law could reveal a divide between Labour and Mr Peters.
"You'd say on his track record, as leader of New Zealand First, has always been for that law and opposed to change," Mr Bridges said.
"We'll see the power dynamics of that and who's really in charge... Winston Peters may take a slightly different view."
Mr Bridges position is that the law shouldn't change.
"It's had a very strong deterrent effect. It is working," he said.
Mr Little said the law had not successfully reduced serious crime, and had also not acted as an effective deterrent.
In 2016, Jacinda Ardern, then justice spokesperson, said the law removed the discretion of judges and should never have been passed.