"Knowing what I know now, do I think it was a good idea to go? No I don't."
The sight of Iraqi troops fleeing Ramadi and the increased use of Shia militias to fight Isis had made him rethink his support.
Dr Smith said anti-Isis coalition leadership under the US had been "feeble", limiting the usefulness of the Kiwi contribution.
It was important New Zealand supported its longstanding allies but the deployment was "almost in the nature of gesture politics," Dr Smith said.
"We've got special forces which are reasonably well-maintained, trained and equipped. Beyond that, we've got no defence forces. We've got no air force."
This was paradoxical, given the aerial focus of coalition attacks on Isis so far.
Dr Smith said any upcoming battles would be "very nasty" and he was worried Shia paramilitary forces would have little more respect for the laws of war than Isis had.
NZDF troops have arrived as a regional conflict between Shia and Sunni escalates, with Iran and Saudi Arabia struggling for dominance in proxy wars in Iraq and Yemen.
Most Isis gains lately have been in Sunni-majority areas. After the fall of Saddam Hussein, some long-oppressed Shia quickly exacted revenge on their Sunni neighbours. In return, some Sunnis now supported or tolerated Isis and its fiercely anti-Shia ideology.
Auckland-based defence analyst Dr Paul G Buchanan said Taji's location triggered concerns for Kiwi troops' safety.
"It's in the middle of the Sunni triangle. It's the last point of defence on the northern flank of Baghdad. These guys have just been put into the middle of a hornet's nest."
He said it was likely most of the 143-odd New Zealand troops were on perimeter guard duty, or focused on preventing "green on blue" attacks, when trainees attacked trainers.
He said the proximity of Isis to Taji and to important nearby highways meant soon, Isis could isolate Taji even without attacking it directly.
Dr Buchanan said NZDF troops would then have to rely on being airlifted out, or "fight their way out of the camp" to re-establish contact with Baghdad.
"I'm very fearful now for those troops. I don't know how heavily armoured they are."
An army spokeswoman did not answer specific questions before deadline, instead referring to an earlier press release that made no mention of NZDF defences in Taji, the demographic makeup of trainees, impact of the Ramadi takeover, or the equipment troops had.
Dr Buchanan said Isis probably believed Baghdad was too strong, fortified and Shia-dominated to fall. But the extremists could try suffocating the city by cutting supply lines, pushing the Iraqi government to sue for peace.
Without a more aggressive coalition effort, he said Isis' co-called Caliphate could potentially reach from the Mediterranean to the Tigris.
"So long as they limit themselves to imposing a Caliphate and Sharia law in the places that they control, and don't try to extend... they may be left to it."
He said the oil-rich oligarchies of the Persian Gulf had the most to fear from Isis. These included the Gulf states Prime Minister John Key visited last month and was keen on signing free trade deals with.
