get an O visa to start from the embassy and open a bank account here as soon as you can. then extend it to retirement at immigration. that's what I did and have been extending for 4 years now. also from nz. you will find it hard to open an account here if at all if you are on tourist visa. also you cannot open a Thai bank account from outside Thailand.
do a non O based on retirement in Australia first and you will need 400 or 800 k in the bank for the first extension then the income in a Thai bank every month for at least 12 months. Australian embassy does not do proff of income and has not for many years now