The Auckland Council of Civil Liberties says there is no basis for law changes allowing increased surveillence and passport cancellations.
The Government said the law changes were needed to counter the threat of New Zealanders fighting for overseas extremists.
It said it was monitoring 40 possible New Zealanders engaged in extremist behaviour, and five New Zealand citizens or residents were fighting alongside Islamic State in Syria.
But the council's president, Barry Wilson, said the claims had not been challenged or verified.
He said the Government was rushing through legislation under urgency for no reason.
"Our civil liberties are being impacted by what amounts to legislation by rumour because we're expected to accept the Government's assertion abou the level of support for ISIS at its face value."
Mr Wilson said the Government had fictionalised a terrorist bogeyman to rush through law changes.