Parliament has voted against an amendment to spying legislation that'd see greater opposition representation on the committee that oversees New Zealand's security agencies.

The Security and Intelligence Bill has been hashed out through the house committee stage at parliament this week. The legislation, when passed into law, will have a single Act governing both the SIS and GCSB, as opposed to the two Acts they currently operate under.

It also creates a new regime for SIS and GCSB surveillance warrants. The government claims the new structure will increase oversight of both agencies.

Labour wanted to raise the number of seats on the committee from seven to nine, to include representation from Labour and the Greens. However it was voted down 75-44.

Departing Labour MP David Cunliffe said it should be increased as it would "improve the scrutiny and the buy-in by parliament into the operation of these vital agencies and so will improve, in my view, the quality of the committee's work."

Attorney General Chris Finlayson said he didn't believe the extra representation was necessary.

"You've got the guarantee of the regular periodic review of the legislation and it could well be that a future parliament decides to change the matter," he noted.