A lawyer is warning police monitoring Facebook pages and using the information to catch drug dealers to tread carefully.
Canterbury police confirmed they had been monitoring Facebook group pages as part of a crackdown on the sale and distribution of drugs.
Detective Senior Sergeant Jason Stewart said police became aware of the pages from members of the public or by coming across the pages themselves.
In some cases, the police have sent letters to group members, visited them or searched their property.
He said the searches have sometimes resulted in arrests but he couldn't say how many.
Kensington Swan partner Hayden Wilson said if a Facebook group was a public group, the people who shared information on it could not have an expectation of privacy.
But he said the police needed to take a cautious approach.
"Although the information might not be private, I'd hope that police were pretty careful about identifying who it was they were sending these letters to.
"If they were to get the wrong person, for instance, then they might have some issues; if they were to send the letter to the right person but in a way that it got to someone else, there's potential that they might have issues there as well."
Mr Stewart said the internet was a public space and anyone who was using it to sell or distribute controlled drugs should think twice.
"Our intelligence officers are looking for certain things and are obviously finding it."
He said police recognised addiction to controlled drugs contributed to crime.
"This is about encouraging addicts to seek help and break the cycle of crime... As part of our prevention work in the Canterbury area, we will continue to monitor Facebook pages that are involved in criminal activity."
He said the issue was not a new one.
"Most New Zealanders have embraced social media. Once you get that, there's always an element that will use it for dishonest purposes.
"We've been aware of certain things for quite a while and part of our recent initiatives in either sending letters or visiting people has been going on for the last two months."
He admitted selling drugs online was likely to be a national, or even international issue.
"It's certainly not an issue that's isolated to Canterbury, or even to New Zealand.
"I think when one page gets taken down often another will come so what we're looking at is targeting the administrators or people that are setting them up.
"Then with the people who are using the sites or are members, we're focusing on rehabilitation, getting them in touch with drug rehabilitation services."
A spokesperson for police national headquarters said it was likely other police districts across the country carried out similar operations.
In a statement, the spokesperson said it was not possible to reveal information that would compromise investigations.
"It would be unrealistic to provide numbers of officers or cases across New Zealand that are looking at online sources at any one time.
"Online crime affects many New Zealanders and just because it is online doesn't make it any less real."