Hey all,
I'm currently doing a masterplan for the Miramar Peninsula in Wellington, New Zealand and the place is littered with coastal batteries that were installed in both world wars to protect the harbour entrance. The notable ones were to protect against Japanese invasion forces, although I doubt they would have been of much use.
I have never really heard much about how real the threat of invasion was from Japan, so I was wondering if anyone could shed some light on the possibilities, the potential strategy and contingency plans that the allies had (namely the rest of the commonwealth).
Practically speaking? There was absolutely no threat to New Zealand directly. The Japanese didn't have the capability to supply troops that far. And the Japanese knew it. They were having difficulty keeping troops supplied in New Guinea and the Solomons.
There was a brief proposal from the Japanese Navy for a campaign down the coast of Australia. But the Army nixed that idea as impossible. And that would have allowed them to build and maintain air bases, supply depots, and the like. For New Zealand they would have had to go directly across open water from New Caledonia. Which would have been significantly more difficult than the plan the Army refused for Australia.
All that said though, in early 1942 Allied planners didn't know that. The Japanese were exceeding all Allied pre-war expectations. Pearl Harbor and then the Malaysian campaign were massive surprises. It made the Allies a lot more cautious. New Zealand had a major role to play as a supply depot and a staging area. So it was defended far beyond the actual threat. It also re-assured locals that they were being defended. So it served political purposes as well as practical defense purposes.