Wellington, March 18 NZPA - The acquittal of three activists yesterday on charges of vandalising the Waihopai spy base near Blenheim does not set a legal precedent, but other defendants may consider using the same "greater good" defence, a criminal law expert says.

Adrian Leason, Peter Murnane and Sam Land admitted they broke into Waihopai and slashed an inflatable plastic dome covering a satellite dish, but pleaded not guilty to wilful damage and burglary charges because they believed their actions were lawful.

A jury in Wellington District Court took only two hours to find them not guilty after a trial lasting eight days.

The Waihopai base is operated by the Government Communications Security Bureau (GCSB), which has not yet commented on the verdict.

Murnane said the trio weren't sure their line of defence would success, but they knew what they had done "was right" because it was done for the greater good.

"That brought a claim of right and we knew we had a right to do that because we were saving people from much greater evil than the mere cutting of a bit of plastic."

Law Society criminal law sub-committee convenor Jonathan Krebs told Radio New Zealand he had never heard of a defence referring to a greater good before.

The "claim of right" defence used by the trio was enshrined in statute law, but usually used in property cases.

An example of its regular use would be as a defence when stolen property was unwittingly purchased, with the purchaser believing the seller owned and had a claim of right to the property, Mr Krebs said.

"[The Waihopai defendants] claimed that they honestly believed that they had the right to do so because they needed to, for certain honestly-held beliefs."

No precedent was created by their acquittal, because precedent was only set by judicial decisions on matters of law, not jury decisions on facts, he said.

"I can imagine, as often happens when a defence is raised and receives such widespread publicity and discussion, that others might be interested in at least considering advancing it but...there's actually no binding effect from the decision of the jury."

Green MP Keith Locke said yesterday the acquittal was a victory for the peace movement, which has campaigned for the closure of the base.

"I hope that the not guilty verdict will help break down the blanket of secrecy that successive governments have imposed around the operations of the base, and its true purpose," he said.

The Christchurch-based Anti-Bases Campaign went further, calling for prosecution of the base operators for crimes against humanity.

Spokesman Murray Horton said the "Domebusters" had believed they had the law on their side and were proud of what they did.

"They did it because Waihopai operates, in all but name, as an outpost of US intelligence on NZ soil..." he said.

He said the base should be closed immediately.

Murnane said outside the court, he believed the satellite aided crimes against humanity.

"I had to do this, it was necessary for me," he said.

"We wanted, in going into Waihopai, to challenge these warfaring behaviours and I think we have done this," he said.

"We have shown New Zealanders there is a US spy base in our midst."

Crown prosecutor Glen Marshall said the men deliberately and intentionally damaged the satellite cover hoping, rather than believing, their actions were lawful.

"It's not a belief, it's something less -- a hope, an expectation, an opportunity,"