@Graeme **********
then suggest you either get an O-A visa or Non O visa for retirement while in New Zealand from the Thai Embassy.
O-A allows you to qualify by having proof of 800,000 Thai baht in your bank and or combo of annual income plus savings to = 800,000. Upon approval it gives you one year multiple entry visa to Thailand...and can extend it one more year if you bounce out and return before the end of the first year from date of the OA visa issue.
The Non O as retiree gives you 90 days on arrival and can be extended one year at a time by having proof of 800,000 Thai baht deposited into your Thai bank account in your name only.
To leave and return requires simple reentry permit at Thai immigration 1000 Thai baht single reentry...3,800 Thai baht multiple reentry for the duration of your Non O based retirement extension of stay.