We know when our troops will begin their mission ... but we're not telling you just yet.

That's what the Defence Force is saying over the deployment of New Zealand soldiers to train Iraqi forces in the fight against the Islamic State.

Lieutenant General Tim Keating, chief of the Defence Force, has told TVNZ's Q+A the timing of an announcement will be up to him.

"What I'll do is I'll give the government advice when I think it's appropriate to say that we have people on the ground and we're starting to fulfil our mission in Afghanistan."

When asked about if he knew when our mission would begin, Keating responded with "I certainly do."

He says they've been quite open about what they're there for, and the existence of a nine-month review of the deployment.

Keating believes most people would understand the sensitivities around specific deployment dates.