The GCSB is forbidden by law to spy on New Zealand citizens or permanent residents.
Ofcanz gave the assurance all four were foreign nationals despite Mr Dotcom and his Dutch co-accused Bram Van Der Kolk being permanent residents of New Zealand.
The mistake is the latest in a series of legal missteps by police and other agencies involved in the January 20 raid on Dotcom's Coatsville mansion, including the use of search warrants later found to be invalid and the seizure of Mr Dotcom's cash, cars and property using a court order which should never have been granted.
Yesterday's documents were issued ahead of a High Court hearing in Auckland today which will deal with the GCSB's involvement.
The documents show Ofcanz asked the spy agency to obtain information relevant to the "location, awareness on the part of the wanted person of law enforcement interest in them, or any information indicating risk factors in effecting any arrest" relating to the four men.
Spying on the group began on December 16 last year and ended on January 20 this year but did not involve installing devices.
Detective Inspector Grant Wormald, who oversaw the raid, told the High Court last month of meetings just before the raid and also two months earlier that were attended by what was then referred to as an unnamed government organisation.
Information about the GCSB's involvement was kept secret last month as Acting Prime Minister Bill English had signed a "ministerial certificate" which effectively suppressed it.
Mr Key, who is the minster normally responsible for the GCSB, was in the US watching his son play in a baseball tournament at the time.
Mr Key yesterday would not comment on the incorrect information supplied by Ofcanz, saying he first wanted to see a report on it from Inspector General Paul Neazor, expected by the end of the week.
"I strongly urge you not to jump to too many conclusions at this point."
However, in Parliament yesterday, he faced a barrage of questions from Greens Leader Russel Norman, NZ First's Winston Peters and Labour Leader David Shearer.
Mr Shearer challenged Mr Key how the GCSB could have been unaware of Mr Dotcom's residency given the massive fireworks display in Auckland at New Year's Eve 2010 which was a celebration of that event.
Mr Shearer said it was incredible that the GCSB had not done its own checks on Mr Dotcom's residency. "It's called the 'intelligence' agency."
Mr Dotcom's US lawyer Ira Rothken told the Herald he was awaiting Justice Neazor's report with interest.
"Certainly domestic spying is a very serious matter. It goes to the heart of civil liberties. It is a very deep privacy intrusion and bypasses the judicial mechanism for checks and balances and certainly can lead to Government abuse - so it's certainly something we're taking very seriously."
However, Mr Rothken said he didn't want to pass judgement on the competence or otherwise of the GCSB.
"We're concerned about a lot of Government aggression and misconduct in this case."
- Additional reporting: Andrew Koubaridis
Taxpayer up for millions
The New Zealand taxpayer may face a multimillion-dollar damages claim from internet tycoon Kim Dotcom over the destruction of his Megaupload business should the United States bid to extradite him to face racketeering charges fail.
Megaupload was shut down after a police raid on Mr Dotcom's Coatesville mansion in January.
Attorney-General Chris Finlayson said that "an undertaking as to damages was given by the Commissioner of Police in relation to the foreign restraining order made on Mr Dotcom's New Zealand-based assets".
The Herald has previously reported that Police Commissioner Peter Marshall gave a signed assurance that police would be the agency with legal liability in any damages claim. Mr Finlayson later said the undertaking covered $20.5 million of assets seized.
It is understood Mr Dotcom had Megaupload valued at $1 billion for the purposes of an initial public offer. The Herald understands he may argue that the disruption to access to funding contributed to the Megaupload business degrading to the point of ruin.
However Mr Dotcom's US lawyer, Ira Rothken, said: "The furthest thing from our mind right now has to be theoretical civil claims or money damages."
Mr Dotcom said on Twitter: "I prefer not to sue the NZ Government. We love it here and we don't want to burden taxpayers."
Q&A
What was the unlawful activity?
The GCSB is expressly forbidden from monitoring the communications of New Zealand citizens or residents but it eavesdropped on Kim Dotcom and Bram van der Kolk who were both permanent New Zealand residents.
When did Dotcom become a New Zealand resident?
In December 2010. Dotcom was one of the first foreign nationals to be granted permanent residency under the "Entrepreneur Plus" scheme which favoured wealthy applicants after he invested $10 million in government bonds.
Should the GCSB have known Dotcom and van der Kolk were residents?
It asked the police's Ofcanz unit for assurances that all the people it was being asked to monitor were foreign nationals and was told they were. However Dotcom's successful residency application was widely reported in the media.
When did the eavesdropping happen?Between December 16 and January 20, the day of the raid on Dotcom's Coatesville home.
How did the GCSB's involvement come to light?
Detective Inspector Grant Wormald, the police officer leading the raids, said during a court hearing last month that an unnamed government organisation was present during planning meetings early this year. The GCSB's unlawful conduct effectively voided a "ministerial certificate" or gagging order issued by acting Prime Minister Bill English on August 10 prompting Prime Minister John Key's acknowledgment this week of the bureau's involvement.
