Labour leader David Shearer says the Government Communications Security Bureau legislation is just a random piece of law designed to get the prime minister out of a hole.
Mr Shearer was one of about 1500 people who protested against the legislation in Auckland on Saturday. Rallies were also held in other centres.
The legislation would allow the bureau to spy on New Zealanders and store their information.
Mr Shearer said the public needs to have confidence in the agency and its processes.
He said they want and deserve an independent review of the spy agencies.
Mr Shearer said he will instigate just that if he forms the next Government.
Independent MP Peter Dunne says people have the right to protest against the legislation governing the spy agency.
Mr Dunne had been holding back his support, but changed his mind after the Prime Minister agreed to make changes to the bill.
Those include mandating a review of the GCSB and the Security Intelligence Service in 2015 and every five to seven years after that.
Mr Dunne declined to comment further on the protests on Saturday.