Wellington lawyer John Miller made a routine request for discovery of police documents four months ago in a case involving gangs and drugs.
When he opened the heavily censored electronic files, "I could see people getting killed", he said yesterday.
"All the names of the informers inside the gangs, all the blacked-out sensitive information, were revealed as soon as we copied the files across. These gangs are dangerous and I realised human lives were at stake."
Police yesterday continued to downplay the bumbling release of confidential files to defendants charged after Operation Canary - a four-year investigation into large-scale commercial cannabis growing in Southland - insisting it was an isolated incident that had not compromised their intelligence-gathering techniques.
But Mr Miller, a respected lawyer and former Victoria University academic, thinks police carelessness is threatening their whole informer programme.
Mr Miller said his son Jonathon, also a lawyer in his practice, had made the discovery on a Friday and he had wrestled with what to do all weekend before advising the Crown.
"They were very grateful."
He believed some defence lawyers would criticise that decision because his responsibility was to his client. "But in all consciousness we couldn't have done anything else [but hand the insecure files back].
"Our law practice is secure but this was a dangerous situation and I wasn't sure the same security existed at other firms. When we give our clients copies of police files we print hard copies for them but I'm aware it is common with some lawyers to hand over electronic copies."
It had been appallingly easy to expose the blacked-out information.
Mr Miller said he couldn't understand why police were still sending out electronic files with flawed security.
"I would have thought a memo to all stations around the country after I approached them last year would have fixed this and I was surprised to read there are other cases. Who would give information to police if they are going to be exposed like this?"
Mr Miller said he had not bothered checking other redacted police files sent to his firm in old cases but would probably revisit them if they could be related to any new cases he took on.
However, Christchurch lawyer Steve Pollo, who represents seven members of Nelson's Red Devils motorcycle gang charged with drugs and firearms offences, was yesterday checking through the censored police files sent to him under discovery for the hearings.
A former police officer, Mr Rollo said that even though he was a defence lawyer he understood there was a need for police informants and they needed to be protected.
"This sort of sloppy work is going to send shockwaves through the informant programme," he said on Radio New Zealand Checkpoint last night.
"They need strong guarantees they are going to be protected".
Southland police district commander Lane Todd said yesterday that no undercover police officers had been used as part of Operation Canary.
- The Southland Times