It took the Government Communications Security Bureau eight months to discover it had spied on Kim Dotcom illegally, and his lawyer wants to know if the US was involved.
Prime Minister John Key has confirmed in Parliament the agency told him on September 17 about its surveillance.
"The GCSB recognised the error a few days before it told me," he says.
The spying began on December 16 and ended on January 20 - the day Dotcom and three of his associates were arrested in a police raid on his Auckland mansion.
Opposition MPs say it's unbelievable the GCSB took so long to find out Dotcom and one of his associates, Bram van der Kolk, were New Zealand residents.
The GCSB is forbidden by law to spy on New Zealand citizens or residents.
"Google `Dotcom residence' and you get 10 million hits," Labour's deputy leader Grant Robertson says.
"And we have to believe our spy agency is so incompetent it couldn't even use Google to identify that Kim Dotcom was a resident."
MPs also say its "incredible" that the GCSB was unaware of Dotcom's $500,000 fireworks party in 2010 to celebrate gaining residency.
Mr Key, who is the minister responsible for the GCSB, says he has had 15 meetings with the agency this year but the surveillance apparently wasn't mentioned at any of them because he wasn't aware of it until it came to him and confessed it had broken the law.
GCSB’s links with US questioned
Meanwhile Dotcom's lawyers want to know whether the United States was involved in the GCSB's illegal spying on the internet tycoon.
American lawyer Ira Rothken, who is working with Dotcom's defence team, says "all the details" of the Government Communications Security Bureau's surveillance must come out.
"We want to learn what happened, when it happened and what they acquired," he told TV One last night.
"As part of the inquiry, we're looking forward to learning whether or not the United States was involved."
Mr Rothken says it seems from Prime Minister John Key's statement on Monday that the GCSB was gathering information for the police raid on Dotcom's Auckland mansion on January 20.
Mr Key was asked on Wednesday whether the GCSB passed on any information to US authorities.
He said it was connected with the arrest of Dotcom and his associates, and had nothing to do with the extradition case.
The FBI wants to extradite Dotcom to face charges of internet piracy, with proceedings due to start in March.
NZN / 3 News