How can an Australian convert a tourist visa to a Non-Immigrant O visa for retirement without an embassy income letter?

Plain and simple if you don't get the embassy letter (which the Aussie consulate stopped back in the beginning of 2018) you can't use monthly income method to get the initial 90 day Non-O visa issued OR for the first yearly extension.

You need to transfer a minimum of 65K baht a month into a thai bank account in your name only each month, every month for the previous 12 months before you apply for a yearly extension using monthly income method if your consulate doesn't issue the affidavit notary letter.

SO as you can see, those requirements preclude you from using monthly income to apply for the 90 day Non-O visa AND the first year retirement extension two months later.

You can only do it with banked money the first year.

NOW you can get the Non-O with banked money, then the yearly extension with banked money and THEN start transferring money in every month for the next year which would let you get NEXT YEAR'S extension using monthly income.

BUT

Sadly there is no way to do it for the Non-O or the first year extension 😕

You get here, you open a bank account and you transfer IN to the country from abroad 800K baht. Then you can go apply for the initial 90 day Non-O for 2000baht and after that once the money has been in the account for 2 months you can apply for the year extension for 1900baht