Ability of the security intelligence service

The GCSB carried out domestic surveillance for the SIS, police and other agencies until last August, when the Kim Dotcom case raised question marks about its legality.

The new bill will clarify that it is legal for the GCSB to spy on New Zealanders on behalf of another agency nuskin group, if they have a warrant.

Mr Key says if the GCSB doesn't do that surveillance, its technology and skills would have to be replicated in the SIS at "significant cost".

He's ruling that out as an option - despite admitting he hasn't asked for any costings.

"I can't see the point in doing it," he said.

Labour leader David Shearer believes the government is moving to merge the GCSB and SIS - which would give the GCSB and its overseas partners access to surveillance of New Zealanders.

"They have already moved into the same building so it's only natural that that's where the direction will go nuskin," he said.

"This bill will enable that to happen much more easily."

Mr Key said he considered a merger when he first became prime minister, but now believes it is unnecessary.

"We're merging a lot of their support services [including HR], and I think that's probably a better way of going than merging the two organisations in totality."

The bill, which has the backing of National's 59 MPs and ACT's John Banks, nu skin hk needs one more vote to pass.

The government is now eyeing a deal with NZ First to get the bill across the line after United Future's Peter Dunne distanced himself from supporting it.