Labour leader David Shearer says the proposed GCSB bill is "bad law" and would be repealed if the party was successful at next year's election.
However, he said the law would not be rolled back until an independent inquiry into New Zealand's intelligence services was carried out.
The comments follow a public meeting last night at which prominent academics and lawyers called for the GCSB bill to be canned.
Mr Shearer said the legislation - which he believed was likely to pass - would be left in place to govern the Government Communications Security Bureau while the inquiry was carried out.
"Otherwise you'd end up having a rogue agency operating without any legislation."
But he said the bill's days would be numbered from the moment Labour came to power, were it successful in the next election.
"We would have an independent review and legislation would come out of the review. The current legislation would need to be repealed, modified or whatever."
The inquiry would begin "immediately" after the election, if Labour formed a government. "That would be one of the first things we got off the ground."
He said it was hard to imagine the bill not going through after United Future leader Peter Dunne pledged his vote to the government.
Mr Dunne yesterday told the Herald one factor in his decision to commit was concern NZ First leader Winston Peters would cut a deal first, and offer few changes to the bill. Mr Shearer said the changes made a "bad law" marginally better but did not meet his concerns over it.
The changes National agreed with Mr Dunne were panned by Dr Rodney Harrison QC and Tech Liberty's Thomas Beagle at the public meeting on the bill.
The lawyers said they made little difference to a law change which allowed mass surveillance on the New Zealand public.