Opponents say sending New Zealand's troops to Iraq is playing into terrorists' hands.

New Zealand is sending 143 military personnel into Iraq, including 16 specialist trainers, to train Iraqi soldiers.

Prime Minister John Key says he doesn't think Islamic State will be defeated in two years, but New Zealand will make a contribution and won't stay in Iraq longer than that.

Terrorism expert Professor Richard Jackson says terrorists want westerners to get involved in conflict with them and told TVNZ's Q&A program the West is playing into their hands.

He says it allows ISIS to fight directly against its enemy, and confirms its argument that the West only want to kill Muslims and keep invading.

Meanwhile, NZ opposition defence spokesman Phil Goff says the $34 million being spent to send NZ trainers could make a real difference providing humanitarian aid.