The Government will change the "claim of right" defence so it cannot be used as it was by three activists who escaped conviction despite wilfully damaging a communications base at Waihopai.

In March a jury found Adrian Leason, Peter Murnane and Sam Land not guilty of burglary or wilful damage at the Government Communications Security Bureau base, based on the defence.

The men had cut electric fences and slashed one of two inflatable domes covering satellite dishes, causing $1.1 million damage.

Case notes show that they argued their actions were legal "in order to defend civilians in Iraq, particularly [those] who were being hurt or killed in part through the use of the intelligence that they considered was being gathered, or at least relayed through the Waihopai Base".

It is believed it was the first time such a defence - which rests on the belief that what the defendants did was lawful - was used in this way in New Zealand.

A Justice Ministry report said the law had been applied correctly and the judge had properly instructed the jury on applying the law.

Yesterday Justice Minister Simon Power said the defence had become too broad and should not apply to cases such as the Waihopai case. The current law meant the "reasonableness" of the actions could not be challenged, and the Crown had to prove that the accused did not honestly believe their actions were lawful.

The Justice Ministry report recommended changing or repealing the law.

The ministry will look into the best way forward and report back in September, and Mr Power said the law would be changed.

"It's clear it was not intended to be used to excuse the behaviour of people who take or damage property in cases where they are not claiming a personal property right," he said.

"Concern has been raised that the legislative scope of 'claim of right' has become too broad and that the defence should not be able to be used as it was in the Waihopai case, and I agree with that.

"It also appears our law on this is out of step with comparable overseas jurisdictions, including England, Canada, and several Australian states."

The New Zealand Law Society agreed the law needed to be changed.

"Everybody, including most lawyers, were surprised it was a successful defence in that case," said Jonathan Krebs, the society's convener of the criminal law subcommittee.

"It's quite astounding that the defence could be successful when these vandals decided they were going to go and destroy this thing. That needs to be corrected."

But he said it should not be repealed because it was a legitimate defence in some cases, such as when buying stolen goods that the buyer thought was a legitimate sale.

Green Party MP Keith Locke said reforming the law was a waste of time. "There are a lot of urgent things for the Government to be spending its scarce resources on. Tinkering with [this] law is not one of them."

What is the "claim of right" defence?
* Applies to 14 sections of the Crimes Act (all property offences).

* Rests on a belief of the right to property or money of another, even if that belief is mistaken or unreasonable; the belief must be honestly held and of legal, not moral, entitlement.

* It is up to the Crown to show there is no "claim of right".

* Used by receivers of stolen goods, for example.

Options to change the defence:
* The defendant would have to prove he or she has a "claim of right", rather than the prosecution needing to prove he or she does not.

* The defendant would have to show that his or her actions and/or beliefs were reasonable.

* Removing "claim of right" as a defence from some or all of the 14 offences in the Crimes Act to which it currently applies.

* Only a defendant with a legal claim to the property concerned could use the defence.

* Repeal.