A legal ethics expert says the defence lawyer who was sent confidential police files was not necessarily obliged to send them back.
A standard criminal disclosure sent to the lawyer mistakenly contained a link to intelligence documents about a drugs operation, with details about informants.
The documents were then widely circulated.
A Christchurch lawyer who specialises in legal ethics, Duncan Webb, said the lawyer was placed in a difficult position.
Mr Webb said legally privileged information must be returned unused, but there was no suggestion that applied in this case.
He said although people would assume the files should have been returned untouched, that could be overridden by the lawyer's duty to their client, if the information was useful to their case.
However, he said there was an assumption the information should not be shared beyond the client.
Assistant Commissioner Malcolm Burgess would not comment on the information yesterday but said in a statement that police deeply regretted the mistake, which was the result of human error.
A criminal lawyer who saw the files said they contained details about informants, criminal activity and police surveillance that could put people at risk.
Yesterday Criminal Bar Association president Tony Bouchier said the files were high level intelligence documents about a methamphetamine operation.
"It is information about informants, it's intelligence-gathering information about criminal activity, it's also information about court orders police have obtained and listening warrants."
The police tried to recover the information as soon as they were aware of the error, but it had already been widely circulated.
Mr Bouchier said it was likely criminals had seen the files, putting peoples' lives at risk.
"I think there there is an absolutely definite possibility that there are lives at risk because they are talking about information about informants and, as I saw just the few pages of documentation that I read, it wouldn't take long for criminal groups or gangs to put their heads together and filter out who those informants might be."
Mr Burgess said the police had taken steps to prevent harm to anyone at risk.
He said police took the security of information very seriously and had reviewed and amended disclosure processes as a result of the incident.
This had resulted in changes to both policies and training for those involved in disclosure.