Saving money and convenience should not be the reason defendants lose their right to a trial by jury, the Human Rights Commission says.

Commissioner Judy McGregor said changes in the Criminal Procedure (Reform and Modernisation) Bill did not appear to be prompted by public demand and figures provided on how much the changes would save were unclear.

The commission supported parts of the bill but said elements could undermine the justice system.

"Paramount among these is the decision to limit jury trials to those carrying a sentence of three years or more and increased reliance on judicial decision making in a variety of areas."

The bill required a change to the Bill of Rights Act, which Dr McGregor said cast doubt on the status of the law.

Proper public consultation should be sought before changing the threshold for jury trials.

"Parliament needs to be asking whether the Law Commission and Ministry of Justice websites constitutes true public consultation if you want a genuine understanding by the public and involvement in the democratic process.

"We cannot see a justification other than costs and expediency for changing the threshold in this way."

She said for some people there would be serious consequences, such as losing their job, for a conviction on a minor offence.

National's Simon Bridges said the line drawn for when a trial was by jury or judge was arbitrary anyway.

Dr McGregor agreed.

"In terms of people's human rights we believe a lower threshold, the low threshold we have at the moment, doesn't pose significant difficulties other than cost and expediency difficulties.

"If that is to change I think there needs to be a decent public consensus about it and I don't think there is at the moment."

Commissoner Jeremy Pope argued that the same level of justice wasn't offered in judge only cases.

"When you have a single judge sitting then you are limited to the life experience of a single judge."

He knew of a judge who was hard on shoplifting cases until he once accidentally left a store without paying.

"It was a complete revelation to him, and he adopted a completely different approach in shoplifting cases thereafter."

Mr Bridges said many High Court cases were heard just by judges, despite having serious consequences.

Mr Pope said he wasn't saying there couldn't be changes but they had to be properly justified.

"Of course you can change but it's a question of you responding to public consultation that shows a wish for it to be changed."