More than 3000 defence personnel from 12 countries are taking part in New Zealand's largest military exercise throughout the upper South Island.
Exercise Southern Katipo, backed by amphibious ships, aircraft and armoured vehicles, has been designed to test the ability of combined forces to plan and conduct joint operations involving sea, land and air.
Lieutenant colonel Martin Dransfield said the scenario was one where New Zealand leads a coalition force into a fictitious country, to restore law and order and provide humanitarian aid.
He said the upper South Island - Westport, Greymouth, Nelson, the Nelson Lakes, Kaikōura, Ward and the Marlborough Sounds - had been cast as the "troubled" Becara region in the South Pacific.
He said it was chosen because it provides challenging terrain that tests forces to a high degree.
The exercise would include intelligence gathering, conducting surveillance and reconnaissance, evacuation of civilians, delivery of humanitarian aid and maritime patrols, peacekeeping and conventional warfighting.
Lt Colonel Dransfield said past experience had proved the training invaluable. He said it was only months after the last similar exercise in 2015 that forces were able to put their training to good use in Fiji, helping in the aftermath of a cyclone.
The defence force said the exercise involves hundreds of residents from Westport, Greymouth, Kaikōura and Havelock who have been "tapped" to play protesters, "displaced Becarans" and foreign nationals in need of evacuation.
The exercise includes personnel from the Pacific, Europe and the United States, and runs until 18 November.
New Zealand Customs, the Ministry of Foreign Affairs and Trade, police, Ministry for Primary Industries, Immigration New Zealand, Ministry of Health and New Zealand Transport Authority are all supporting the exercise.