The police-paid spy Rob Gilchrist passed officers information about an anti-Taser rally organised by the Green Party.
The Greens say Mr Gilchrist also infiltrated their parliamentary offices while working for the police - which the party says could breach the rules about how police act in Parliament's precincts.
But a police spokesperson said those rules applied to officers only - not to a private citizen like Mr Gilchrist.
Mr Gilchrist was caught out after his ex-girlfriend Rochelle Rees discovered emails he forwarded to police, such as one from 2006 where a Greens staffer circulated information about a Taser protest rally to take place on Parliament's steps.
Mr Gilchrist's email subject line said "Hehehe - Look at the source of this :-)".
Greens MP Keith Locke said the forwarding of such emails showed police were conducting surveillance "with an antagonism to lobbying".
"The police wanted the Taser and perhaps wanted to monitor and counter the Green Party's opposition and even went to the extent of using their covert agent to gather information to that end," said Mr Locke.
"The role of the police is to be politically neutral - not to monitor an elected political party for the police's own purposes."
Other Green Party emails that Mr Gilchrist forwarded included press releases by MPs Metiria Turei and Sue Kedgley and a note about a GE-free meeting.
Mr Locke said he was concerned that former MP Nandor Tanczos' secretary Rayna Fahey had contacted the party to say that Mr Gilchrist had "ingratiated" his way into the offices before the 2005 election.
Police behaviour in Parliament is governed by the agreement called "Policing Functions within the Parliamentary Precincts", signed by former Speaker Jonathan Hunt and former Commissioner Rob Robinson in June 2004.
The agreement said Parliament's buildings were not a sanctuary to the law, but "nevertheless, the police should exercise their powers having regard to the authority of the Speaker and privileges of the House. Parliamentary business and members of Parliament must not be subject to any improper interference or pressure."
Police were able to come into Parliament without the Speaker's permission to make an arrest or stop a crime, but had to inform the Speaker as soon as possible afterwards.
Mr Locke said it did not matter that Mr Gilchrist was a private citizen, as he was being paid $600 a week as well as receiving free travel and accommodation.
Mr Locke said it was obvious that the protocol did not extend to private citizens acting as police informants, but in doing so, police were not acting in the spirit of the agreement.
Mr Locke said he would be writing to Prime Minister John Key, Police Minister Judith Collins and Police Commissioner Howard Broad with his concerns.
A spokesman for Speaker Lockwood Smith also pointed out Mr Gilchrist was a private citizen and said it was a matter for the other parties involved.