I'm interested that you trace NZ's current policy back to World War 1. I'm sure there were strong pacifist sentiments in NZ between wars. But pacifism and isolationism were a common reaction to WW1 - think of the efforts Britain took to avoid war with Germany prior to WW2, or indeed the fact that the US didn't join the war till attacked in 1941. None of that stopped young Brits, Americans, Kiwis, Aussies, Indians et al serving their countries with distinction from 1939-45, and for many of those countries again in Korea and Vietnam.
I don't agree that WW1 "soured" NZ on Britain. As he declared war in 1939, our Prime Minister said we "range ourselves without fear by Britain. Where she goes, we go; where she stands, we stand." Sound sour to you? I'd be tempted to look much later for the psychological break - probably to Britain joining the EEC in the early 1970s.