While it was unlikely that would happen, the fact it was possible meant the legislation had to be rejected, he said.
Senator Brown has already called on Labor to back five amendments he plans to introduce to the Bill.
Labor flagged a dozen amendments of its own last week when the Senate began debating the package of five Bills, drawn up in response to the September 11 terrorist attacks on the United States.
"Labor's far too weak on it," Senator Brown said on ABC radio today.
"It's very dangerous legislation.
"It moves us into the potential for a Government that acts like a police state, for example, any protest which takes place which could threaten the health and safety of unspecified people can lead to the people in that protest being labelled a terrorist.
"When you take a literal view of that it could include nurses who are picketing and not attending to hospital beds."
He admitted such a move was unlikely but said it put at risk the traditional freedoms of Australians.
"We don't need this legislation," Senator Brown said.
"We've got very strong legislation to deal with criminals and with terrorists at the moment."
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