The meeting was chaired by retired Supreme Court judge Justice Sir Ted Thomas, who tongue-in-cheek pledged neutrality on the "iniquitous, scurrilous and foul bill".
Mr Dotcom said he expected a change of government would be needed to reverse the law, which is likely to pass with a one-vote majority now United Future leader Peter Dunne has pledged his support.
Labour MP David Cunliffe sat in the front row last night. His party leader, David Shearer, watched unnoticed from the rear of the hall with Labour's finance spokesman, David Parker.
"I am a living and breathing example of why the GCSB must not be given greater powers and limited accountability," Mr Dotcom said.
Referring to the special warrant signed to keep secret the illegal spying against him, he said government actions had been telling.
"[Their] first response was dishonesty, followed by an attempted cover-up.
"The new GCSB bill is like raising the speed limit after getting a speeding ticket. And it doesn't mean the GCSB won't be speeding again."
Dame Anne Salmond said the proposed spy law was one of a series - the others were passed recently - which threatened the rights of New Zealanders. She said Kiwis shouldn't suffer an "electronic McCarthyism" and called on politicians to show "backbone".
"At times like this they need to show some backbone and prove they are worthy of the trust we place in them."
