Those signing the document are asked to provide the passwords so that police can access the accounts and use the information stored on them.
"I consent to the use of my online identity and accounts for any purpose relating to an official investigation by the New Zealand Police," the form says.
Those signing the form "relinquish all present and future claims to the use of these accounts" and are told police will change their passwords so they no longer have access.
The New Zealand Bar Association and the Auckland District Law Society have written to Police Commissioner Andrew Coster, expressing their concern about police trying to assume people's online identities.
"Given the nature of the form and the concerns it raises, we are hoping for an urgent response," Queen's Counsel James Rapley said.
Rapley, the chairman of the New Zealand Bar Association's Criminal Committee, said he would wait for Coster's response before commenting further.
Auckland District Law Society president Marie Dyhrberg QC has also put her name to the concerns expressed to the Police Commissioner.
Defence lawyers contacted RNZ after one discovered the form by chance while working on a client's file.
Criminal Bar Association president Fiona Guy Kidd said she was also concerned about the police tactic.
"It is often very vulnerable people who are being asked to sign these."
Police were asking for broad and open-ended access to social media accounts which contained large amounts of personal information, she said.
"It allows them to pretend to be that person," she said. "Do people actually understand what this is permitting? Do people understand they have a right to say no. What advice are they getting?"
Guy Kidd said while the form asks for consent, that had to be seen in the context of the power dynamic between police and a suspect or defendant.
"It could well come in a situation where a young person is being interviewed about their part in a crime and that is a high pressure situation."
Detective Inspector Stuart Mills from Intercept and Technology Operations said the form was legal, and police had used it on a "regular basis" since 2012, primarily in child exploitation investigations.
"We obtain the consent of the victim and at times the offender to help us identify other offenders," he said, with the aim of finding any ongoing offending by a wider network. Information was shared with the police's New Zealand and international partners.
"Our domestic partners as well as our international partners use the same or similar processes. We obviously have legal advice around the legality."
The form was also used on other types of investigations, he said.
Asked if police could use an online identity indefinitely, Mills said "you could say that, however the consent is used for the specific investigation purpose and then we would progress from there.
"Due to resourcing and other priorities, once the investigation has progressed as far as possible or for the specific purpose then we would obviously go on to something else because we have a huge demand in this area."
