Act Party leader David Seymour says a pattern is emerging in which judges are choosing not to give the harshest penalty for murderers under the three strikes legislation.
On Thursday, Hamilton man Turei Rawiri Kingi, 26, was sentenced to a minimum of 13 years' jail for bludgeoning an elderly man to death with a single strike of a bottle in August.
The murder was Kingi's second "strike", or serious violence offence.
Under a provision in the three strikes legislation, Kingi faced a life sentence without parole -- unless the judge considered this punishment "manifestly unjust".
Justice Edwin Wylie ruled that the sentence would be manifestly unjust because of Kingi's mental health problems and a number of other factors.
It is the third time in three cases that judges have used this clause to avoid the harshest penalty.
The Crown is appealing the first two cases. It is not yet known whether it will appeal Kingi's sentence.
Mr Seymour said he respected the judges' decisions.
But he said that the drafters of the three strikes law would probably be surprised that the "manifestly unjust" clause had been used in 100 per cent of the relevant cases.
He said the safety valve clause was designed to prevent injustices in the most extraordinary, "out-of-the-box" homicide cases in which a life sentence without parole would clearly be wrong.
"It is interesting just from a policy point of view to note that so far this ... scenario has played out three times, once in Auckland, once in Wellington and once in Hamilton, and each time the judge has said that this is one of these ... extraordinary cases," Mr Seymour said.
He did not believe that judges' rulings meant the legislation was too punitive.
"As this policy evolves and the Crown challenges the previous decisions, I'm sure we'll get to an equilibrium. But by and large, we're pleased that the three strikes law is having an effect on sentencing in most cases."
Justice Wylie said a life sentence without parole would be unjust because Kingi was still young and had lived a crime-free life until he began displaying mental health disorders while in the NZ Army in 2009. He had also pleaded guilty, expressed remorse and had not committed "the worst kind of murder" -- which the three strikes legislation targeted.
Analysis by the Law Society showed that after five years, the three strikes law change appeared to be having a deterrent effect, though this was not conclusive.
Ministry of Justice data shows that there have been 5382 first strike warnings and 76 final warnings since 2010.