"88 is about to become 4.4 million," warned trade unionist Dion Martin, saying that the bill would mean journalists could no longer guarantee the anonymity of their sources.
He suggested that the millions of dollars it would take to enforce the new regime would be better spent creating a more equal society, which met with applause from the crowd.
War veteran and former city councillor Peter Wheeler expressed disappointment that people like him, who went overseas to fight for democracy, should find it being eroded at home.
"The people who hold power must be held accountable, not just to themselves, but to all of us," he said.
Central to opposition to the bill is its treatment of "metadata", information incidental to a phone call or email, such as when it was made or who it was to. Under the proposed law, the GCSB would not require a warrant to gather this. Advocacy group Tech Liberty identified metadata as potentially much more useful to intelligence agencies than the contents of the call or email itself.
This cut no ice with protester Sapphire Davis, as she and her family joined the march around The Square.
"I'm here to support everyone in New Zealand protesting against this. We're here to protest for our rights, our privacy." She didn't believe the law would be applied consistently. "Are they going to do it to the politicians? I don't think so."
Another protester was visiting from Australia. "I don't mind cameras in the streets to stop crime, but there shouldn't be cameras in every home," she said. "New Zealand is selling its soul."
The bill has yet to pass its second and third readings but, with the support of Peter Dunne, the Government has the numbers to make it law.
TV3 news incorrectly reported that marchers threw missiles and clashed with police. A retraction was issued during Monday night's 6pm news bulletin.
See editorial, page 2