The Green Party says police have gone beyond the limit of their powers, and have likely broken the law themselves by receiving information from an apparent spy.
Animal activist Rochelle Rees claims her former partner Rob Gilchrist spied on her and her friends in activist groups and passed the information to counter terrorism police.
Ms Rees says she found e-mails to police when she was fixing Mr Gilchrist's computer.
Green MP Keith Locke wants an inquiry into the Police Special Investigation Group and says the Greens will raise the issue in Parliament.
He says the police need to be brought to account.
Mr Locke says people should be encouraged to be engaged in protest activity, not discourage them with police spies.
He says it's dangerous for police if they are targeting peaceful groups.
Police will neither confirm nor deny the existence of any informant in any activist group and say they have never commented on whether Mr Gilchrist was passing information to them.