Prime Minister John Key says he and the head of GCSB would resign if the spy agency were found to have conducted mass surveillance.
He made the comment to reporters at Parliament in the light of assurances that the changes to the GCSB Act 2003 would not mean mass surveillance of New Zealanders.
Asked if he and GCSB chief Ian Fletcher would resign if there were mass surveillance, he said yes.
"But the facts of life are it won't happen."
For that to happen, the GCSB would have to undertake illegal activity.
He clarified later saying "both" would resign if there was mass surveillance.
"If I wholesale blatantly flout the law as Prime Minister I'm never going to survive anyway."
The Government Communications Security Bureau and Related Legislation Amendment Bill will be debated in Parliament this afternoon.
Labour plans to move an amendment to try to get written into the law a policy statement by Mr Key last week in which he said he said he would not grant warrants to the GCSB to look at the content of New Zealanders communications under the cyber security function in the first instance, but if the agency detected a serious cyber intrusion, it would have to come back to him for a second warrant.
Labour would require the leave of the House to introduce such an amendment because the part it relates to has already been dealt with.
Mr Key indicated that National would oppose leave for Labour to do that, saying it was not necessary.
Meanwhile, Justice Minister Judith Collins has confirmed that the police in the past have used the GCSB's specialist capability to intercept the communications of paedophiles.
Such assistance to other agencies has been on hold since September last year, pending the current bill passing which will unequivocally give the GCSB the legal power to spy on New Zealanders in certain circumstances.