The Government is looking at how to deal with jihadists who return from fighting in the Middle East, as part of its action plan against Islamic State.
New Zealand had to look at its law as it was subject to a United Nations Security Council resolution passed last month which targets radicalised jihadists and their supporters, Mr Finlayson told Checkpoint.
Any law changes were being considered carefully and urgently, he said.
They included developing and implementing prosecution, rehabilitation and reintegration strategies for returning foreign terrorist fighters.
"So I would think there will are probably two classes, one would be those who got over there and said 'gee, this isn't what we bargained on, we need to get out of here and get back to New Zealand', and that's fine and we're here to help such people," Mr Finlayson said.
"Secondly, those who decide to come back and promote jihad on these shores, and then the issue really is does the ... Terrorism Suppression Act cover this sort of thing?"
Mr Finlayson said people could not simply be locked up, and that a substantive offence had to be proved.