That's due to rural depopulation, which is a rather different problem from lack of housing in desirable cities. And Japan has, more or less, solved the housing affordability problem in cities too.

Well Japan’s population is decreasing at 0.5% per year at present.

It is pointless trying to compare a country with population growth against a country (like Japan) where housing availability is increasing due to population decline.

Greater Tokyo's population is increasing: https://en.wikipedia.org/wiki/Tokyo#Demographics

It has not had a decrease in population since 1995 and the latest data point (2020) shows an increase of +3.5% compared to 2015.

Not in Tokyo - which what I was talking about!

You can say the same about Germany (and probably most western countries) - vast rural (or depressed urban) areas are depopulating because people move to Berlin/Munich/Hamburg/Frankfurt/etc. And yet Japan solved affordable housing to a much greater extent than us.

But watch the dynamics in New Zealand: retirees sell up in expensive places and retire to cheap places. That is the driver for house prices in lots of places outside of Auckland? The same thing must happen with retirees moving away from Tokyo?

Japanese invest in other things not in their housing (as housing is cheap, they can). Also I believe retirees do not as a rule move away from Tokyo? Here in Germany it's neither expected nor common for people to leave the cities they live in when they retire.

It’s common for urban Japanese to have a big old family home waiting for them in the countryside when they retire.