Auckland's Town Hall filled with hundreds of supporters to hear legal experts and Opposition politicians speak out against the GCSB bill tonight.

The Government Communications Security Bill is expected to pass its committee stages and third reading this week with a one-vote majority.

Speakers took to the stage for seven minutes at a time to explain why they believed the Bill was flawed and unnecessary.

"Well this is what democracy looks like'', MC Martyn 'Bomber' Bradbury told a Town Hall at full capacity.

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"Tonight we hear the other side of the argument.''

Dr Rodney Harrison QC said Prime Minister John Key's comments that New Zealand was not sleepwalking into a surveillance state were flawed.

While Mr Key wanted to leave a strong economy in his legacy as Prime Minister, he did not care about the society he left behind, Mr Harrison said.

Internet tycoon Kim Dotcom said the legislation was like a Hollywood blockbuster spanning different genres; first action, then drama and now a comedy.

"We're being lied to. We're being fooled into thinking this GCSB bill here is to protect us.

"We have a prime minister in New Zealand who thinks he can just push this through with one vote against the will of New Zealanders.''

The public had the opportunity to change that at next year's general election, Mr Dotcom said.

Investigative journalist Jon Stephenson said he found it hard to trust the National Government after finding his moves had been tracked while reporting in Afghanistan.

Weak excuses about weapons of mass destruction and al Qaeda were not a good enough justification to pass the Bill, he said.

Orcon founder Seeby Woodhouse said the National Party mistook George Orwell's dystopian novel 1984 as a guide book.

While New Zealand was rated as the "free-est country in the world'' that would change if the GCSB bill passed, he said.

"We must lead the world ... we must do it again.''

When Mr Bradbury announced a Campbell Live poll showing 89 per cent of New Zealanders were against the Bill the audience at the town hall erupted into rapturous applause.

"You my friends, you are the majority,'' he said.

CTU president Helen Kelly said New Zealanders had the right to expect their private communications to remain private.

"Well we at the trade union have stuff to hide. We have plans, trade secrets ... and this makes us part of a functional democracy.''

It was the State's job to protect that right, Ms Kelly said.

It was ironic that United Future Leader Peter Dunne was planning on voting to pass the Bill when he expressed such outrage at his own communications being monitored, she said.

Labour Party Leader David Shearer said the Prime Minister should back up claims made to the Herald on Friday that he would restrict warrants granted to the spy agency so it could not initially look at the content of New Zealanders' communications by writing it into the Bill.