Any plan to commit New Zealand assistance to US-led efforts against Islamic State (Isis) militants in Iraq will likely be debated in Parliament but the ultimate decision is the Government's alone, Prime Minister John Key says.

Mr Key has not ruled sending the SAS to Iraq but this morning said he was likely to get advice on the options for New Zealand's assistance this week.

Key: SAS could join Isis fight on ground

His executive would make a decision over what to do in the next couple of weeks, but not at the first Cabinet meeting.

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Mr Key said New Zealand's decision would probably pre-empt any formal US request for particular assistance.

"My preference would be to move to a position where New Zealand determines what it's going to do, if anything, and then takes those steps as opposed to someone particularly asking us to do things."

The process of making that decision "could lead to a Parliamentary debate" and Mr Key said it would be his preference to have backing for any assistance from across the House, "although often it will depend on what steps that we took on who was likely to join us".

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Key: SAS could join Isis fight on ground

"In a Parliament that's as diverse as our one you're probably never going to get every political party and Member of Parliament supporting whatever steps you take."

Mr Key said the decision was ultimately for his Government to make and he wouldn't be seeking a mandate from Parliament on potential military assistance.

Sending troops to Iraq was "at outer edge of what I'd like to do".

There were "other options that were far more appropriate and realistic".

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Meanwhile Labour MP Phil Goff says any law changes around New Zealanders travelling overseas to fight should distinguish between those fighting against oppressive regimes like that of President Bashar al-Assad's in Syria and those joining groups guilty of war crimes or terrorism like Isis.

Mr Key has signalled he will soon unveil law changes to tackle "glaring deficiencies", around the cancellation of New Zealand based foreign fighters' passports.

He said yesterday he wanted to reform SIS legislation, preferably with Labour's support.

Mr Goff said there would be a case for strengthening legislation to deal with people intending to fight for groups guilty of war crimes and terrorism, "but you've got to be careful that you don't use legislation to have a blanket suppression of those who are simply aiming to fight alongside friends and family against an oppressive regime.

"I think you can distinguish between the two different circumstances and ought to do so."

Mr Goff also said Mr Key was offered Labour's support for GCSB law changes last year, "but only made a token gesture toward it", so he was sceptical that Mr Key was now seeking Labour's support for SIS legislation changes in good faith.

Labour was open to discussing improvements in SIS legislation as long as there were safeguards against the abuse of power but it still wanted a wide-ranging review of New Zealand's intelligence and security apparatus.

Green Party Co-leader Russel Norman said Mr Key's handing of key aspects of his intelligence and security work to Attorney General Chris Finlayson as part of yesterday's Cabinet reshuffle were "a clear admission of his own failings of oversight''.

"It's a tragedy that he didn't do it earlier."

He said Mr Key's suggestions of law changes around foreign fighters seemed an extraordinary step and part of a pattern seen overseas ``in terms of the very heightened level of rhetoric coming out of particularly Britain but also Australia.

"We'll see what's in the legislation as to whether it's really necessary or whether it's more a political instrument."