The Engineering Printing and Manufacturing Union (EPMU) has added its weight to calls for a commission of inquiry into police spying.
The EPMU said it backed today's call by Unite Union for a full public inquiry into the activities of the police Special Investigation Group (SIG) following revelations a paid informant for the unit was spying on union industrial and political campaigns.
Earlier this week Prime Minister John Key ruled out an inquiry and said police assurances had been given to the Government that covert investigations were justified.
The EPMU said emails it had obtained showed paid informant Rob Gilchrist forwarded to police meeting times and venues for the union's campaign against the National Party's original bill in 2006 to allow workers in small businesses to be sacked within 90 days of starting work.
Mr Gilchrist also sent the schedule for union pickets during the Progressive Enterprises lockout, which involved 120 EPMU members, it said.
The emails also showed the SIG received information on at least seven other union groups, including Unite, the Service and Food Workers Union, the National Distribution Union, the Maritime Union, the National Union of Public Employees, the Youth Union Movement and the Council of Trade Unions.
EPMU national secretary Andrew Little said the SIG's actions were well outside its mandate to monitor terrorism and threats to national security and a commission of inquiry was needed to get to the bottom of it.
"For the sake of public confidence it needs to be established whether Gilchrist was acting as a police-sponsored unguided missile or whether something more sinister is afoot," Mr Little said.
"In a free and democratic society, citizens have the right to organise and protest and demonstrate and should be able to do so without being molested by the police."
Earlier Unite leader Matt McCarten released emails from Mr Gilchrist to the SIG detailing activities being organised by the union as part of their push to abolish youth pay rates and increase the minimum wage.
Reports of police spying on protest groups broke at the weekend.
Rochelle Rees, an animal rights and Labour Party activist, told the Sunday Star-Times she had discovered her former partner, Mr Gilchrist, had been paid by counter-terrorism police to spy on the protest groups when she helped him fix his computer.
Green MP Keith Locke called for an inquiry when the story first broke, and yesterday his party was furious that Mr Gilchrist had infiltrated its offices and was used to report on the party's activities.
Mr Locke said there were rules about how police could act in Parliament's precincts and he would be writing to Mr Key, Police Minister Judith Collins and Police Commissioner Howard Broad with his concerns.
The SIG teams were set up in 2004 to focus on terrorism and threats to national security.
