A private member's bill replacing the verdict of "not guilty by reason of insanity" with "proven but insane" has been stalled after an 11th-hour intervention by Chief Justice Helen Winkelmann.

The sponsor of the bill, Taupō MP Louise Upston, says she is offended by the late intervention after the justice committee had worked hard on refining the bill and unanimously supported it.

Had it not been for the Chief Justice's intervention, the Rights for Victims of Insane Offenders Bill would have passed its committee stages two weeks ago and would have been passed into law today.

But the Government tonight voted for the bill to return to the justice committee to consider the objections by the judiciary.

Upston was ordered out of the House by Speaker Trevor Mallard before the vote for making unparliamentary objections.

Upston told the Herald she sponsored the bill in order to spare victims and families of victims the pain of having to hear "not guilty" verdict read in court when there was no question that the defendant had caused harm.

It was the result of advocacy by people like her constituent Graeme Moyle whose brother Colin, a Good Samaritan, had given a home to a paranoid schizophrenic, Matthew Ahlquist. After row over Ahlquist using drugs, he hit Colin over the head with a spade, doused him with petrol and set him alight.

"This bill is not about Matthew Ahlquist," Upston said in the first reading speech. "This bill is about Colin Moyle, his brother Graeme, their family and every other family of victims like this. This bill is about victims' rights."

She said while changes were made during the select committee process, the objective of the bill remained the same and the judiciary did not make any submission on it until after it had been considered by the committee.

"I think everyone's submissions should be treated equally and they should be done at the appropriate time," said Upston.

She questioned whether the brakes would have been applied to the bill if any other group had objected at such a late stage "and I think the answer would have to be no".

"Then there is the question of is it appropriate for the judiciary to have this level of intervention on the Parliament when it was a unanimous agreement by the select committee.

"I find it, on behalf of the Parliament, quite offensive that Government MPs have dropped their own decision-making and ceded decisions and power to the judiciary and I think that is a very dangerous precedent."

She was not saying the Chief Justice should not be able to express concern, but it should have been done earlier.

The Chief Justice's concerns were raised in a letter to Attorney-General David Parker on May 24.

Justice Winkelmann's letter says the creation of a new verdict of "proven but insane" to replace "not guilty" would present a fundamental change to the criminal law.

And she argued strongly for the current law.

She said the existing law reflected the well-established position that defendants who were legally insane at the time of the alleged offending could not be guilty of the crime they were charged with because the necessary mental element was missing.

"They do not have the relevant 'mens rea' either because they are incapable of forming the necessary intent or because they are incapable of forming the necessary intent or because the intent formed was an insane one.

"Such defendants are not guilty. The charge against them has not been proven because the mental element has not been proven."

She said as a matter of principle, it was inaccurate to use the term "proven" against an insane defendant.

"The charge has not been proven because the mental element – which is an essential element of the offence – has not been proven."

She also said "proven but insane" failed to accommodate the possibility that the defendant had a good defence to the charge even if they did the relevant act but may have done so, for example in self-defence.

She said that change to the law needed to be clarified whether that was an acquittal in respect of the charge.

Upston said she had been working with officials from the Ministry of Justice and Ministry of Health and had come up with amendments to put during the committee stage but it was still being sent back to select committee.

Among the proposed changes is a clause making it clear that in a verdict of "proven but insane" the defendant would be acquitted.

"I actually find it insulting for the member of the committee to be pretty much be told 'you've got it wrong.'"