Opposition parties are renewing their call for a commission of inquiry into the Government Communications Security Bureau now it's been revealed the agency isn't sure whether it spied illegally in three more cases.

And they're attacking Prime Minister John Key for telling parliament he didn't know anything about the Kim Dotcom case until September 17 - the GCSB's files show it was mentioned when he visited its offices in February.

Mr Key says he can't remember any reference to Dotcom during that briefing but Labour, the Greens and NZ First don't accept that.

"John Key has grossly misled parliament," NZ First leader Winston Peters said on Wednesday.

"This just adds more fuel for an independent inquiry into the whole Dotcom fiasco."

Labour leader David Shearer says it seems Mr Key "conveniently forgot" the reference to Dotcom during the February briefing.

"It points to the prime minister not telling the truth," he said.

Greens co-leader Russel Norman says Mr Key is either incompetent or he has misled the public.

"The prime minister has sole democratic oversight of the GCSB but can't remember being told about its involvement in the Kim Dotcom raid," he said.

"It's his duty and his job, spelled out in law, to control the functions of the bureau - his hands-off approach apparently extends to not paying attention when he is told important information."

All three party leaders say the revelation that the GCSB can't give an absolute assurance it didn't spy illegally in three cases other than the Dotcom surveillance proves the urgent need for an independent inquiry.

It has admitted it spied on Dotcom illegally because it didn't know he was a New Zealand resident.

The internet tycoon was arrested on January 20 on allegations of copyright piracy, and the GCSB had been spying on him since December 16.

Mr Key asked the agency to review its files back to 2009 to make sure there hadn't been other bungles.

"In three of 58 cases the GCSB cannot assure me that the legal position is totally clear - more legal work is being undertaken," he said.