However, he said if military support was provided, it would probably be logistical help such as airlift capability rather than people on the ground.
Mr Key said there were risks in New Zealand getting involved - in particular retaliation against aid workers, tourists and New Zealanders on their OEs in Muslim countries in Asia and the Middle East.
However, most of New Zealand's traditional allies were involved to some extent and it would be "odd" if New Zealand did nothing.
The Cabinet will today also consider a proposal to clamp down on New Zealand foreign fighters and Mr Key is expected to soon release the numbers of those believed to be from New Zealand to help boost his case.
He claimed the law changes are required urgently and include possible detention of those suspected of travelling overseas to fight or returning.
Mr Key told Q+A that Cabinet will today look at a proposal for law changes, including issues around passports, which can currently be seized for up to 12 months.
He indicated he was also considering a hard-line change similar to Australia's to detain those suspected of heading off to fight overseas and deal with those returning.
"Potentially, we would have greater powers and potentially even powers to look at arresting someone under the view that they would undertake what would then be deemed to be a criminal act. So that's a very big step. I'm not saying we will take that."
Speaking on Newstalk ZB this morning, Mr Key provided more detail on the Cabinet paper, saying it will set out the terms of reference on proposals to strengthen New Zealand terrorist law, specifically making it illegal for a New Zealand citizen to fight for a terrorist organisation abroad.
"[It] essentially asks the Department of Prime Minister and Cabinet to go away and work with the two intelligence agencies, the SIS and GCSB, to go and do a review of current laws and see if they can identify any deficiencies which would be worthy of a sort of patch, if you like, before a more substantial review of the legislation takes place, beginning no later than June 30 next year," he said.
That review could "run for a year realistically", he said.
"This is saying, 'look at Australia, look at the UK, they've got different rules than us, do we need to have different rules?'," Mr Key explained.
One of the risks he was considering is in relation to people with dual New Zealand and Australian citizenship, he said.
"If Australia has those rules which say, 'if you go and fight for a known terrorist group and you return to Australia, that's a criminal act', with dual passport holders it's pretty obvious to work out that people might well come back to New Zealand and not Australia because they can be charged there," Mr Key said.
However, he admitted such a law was "quite controversial".
"We need to go in an treat this stuff seriously," he said, adding that critics who say there's only a small risk to New Zealand "don't see the stuff I see".
The Government had already cancelled passports of people suspected of planning to leave the country to fight in Syria, he said, describing potential home-grown terrorists as coming from "a range of different backgrounds", with some being "New Zealand-born New Zealanders".
"Some of them if they're in New Zealand are in varying different stages of considering their involvement, but that's anything from people wanting to pack up and go and be a foreign fighter - and some of those people we've been cancelling their passports - and others are looking to finance the operation," he said.
Mr Key said ISIS had risen from nowhere in a short space of time with "a very sophisticated outreach programme", which included social media.
The group's fast rise had "amazed" the US authorities, he said, adding information from both the Pentagon and the White House showed ISIS runs "like a corporation".
Labour's interim leader, David Parker, said his party could not rule out supporting that if Mr Key's evidence showed it was warranted.
He said the case such as the Briton pictured beheading UK and US hostages showed why. "If that was a New Zealander and came back, you would want them to face the consequences and you may want to exclude them."
However, he and NZ First leader Winston Peters questioned the need for law changes under urgency, which meant there was no chance for public submission.
Mr Peters said the PM had not mentioned it as a pressing issue during the election campaign, yet just a month later it was urgent.
The changes are likely to be based on Australia's tougher laws, including its recent Foreign Fighters law to deal with Australians who return from fighting overseas.
A spokesman for Defence Minister Gerry Brownlee said defence personnel deployments this week to the Middle East were not related to the Isis threat.
Q&A
What is Isis?
Islamic State is a radical Islamic group that has seized large territory in eastern Syria and across northern and western Iraq. Its brutal tactics, including beheadings of journalists and soldiers, have sparked outrage across the world.
How did it start?
Isis has its roots in al-Qaeda in Iraq, the Islamic State of Iraq (Isi), and can be traced back to the late Abu Musab al-Zarqawi, a Jordanian who pledged allegiance to Osama bin Laden and formed al-Qaeda in Iraq (Aqi).
What does it want?
It wants to establish a "caliphate", a state ruled according to Sharia or Islamic law by a single political and religious leader.
How did it become so powerful?
Isis has secured huge cashflows from oilfields and Syria and also made large profits from the smuggling of raw materials and antiquities from archeological digs.
Who are its members?
Its leaders are carefully chosen and many who report to the top tier are veterans of the insurgency against the US a decade ago. It has recruited thousands of foreign volunteers and is estimated to have more than 10,000 men under its control.
