Section 9 of the Defence Act 1990 requires the Minister of Defence to use the NZDF for public purposes or assisting the civil power but also requires the Minister to notify the House. Under section 9, the Minister's authority lapses after 14 days unless extended by the House.
I cannot see why a similar provision could not be required for any overseas deployment of the New Zealand Defence Force where force is to be used (ie not on overseas postings, secondments etc). Unlike the civil purpose though, I don't think the Minister should get 14 days free. If the Minister wants the NZDF (or any part of it) to be used to defend "New Zealand's interests" overseas (whatever that means), the Minister must notify the House of this intention and the House must debate it and agree. To make it workable, I guess such an authority would only need to be granted for one continuous engagement. Once authorised, the Minister can authorise subsequent deployments without going back to the House for authority each time. However, when hostilities cease any new engagement including those in the same area would require a new authority from the House.
Currently, there doesn't seem to be any "Declaration of War" mechanism in NZ law. Also, "war" is a term easily skirted around with words like "police action". I think if Kiwis are to be put in harms way, there should be an authority to do so no matter what you call it.
This may represent a fundamental weakening of Executive power. But it seems like a powerfully democratic idea.