Can I use notarized Australian bank and income documents for my annual extension on an OA visa in Thailand?

If this is your first extension of your OA visa then you have to have your Thai health insurance active and in the database before you apply. Your extension will only go until the end of your insurance. If you apply a few weeks early then you will lose a those weeks from your 1 year extension. This only happens the first year. After that you will always have insurance when you apply.

I can just share my experience, I had a retirement visa that I switched to a marriage visa. The immigrantion officer would not accept an official (stamped) Citibank Thailand bank statement - he demanded to see an actual bank book showing the running balance 3 months before and 3months after application. Citibank doesn't have bank books anymore so I needed to open an account at a local Thai bank that still uses bank books. To avoid problems, I suggest doing the same - open a local account and just keep the money (800K for retirement, 400K for marriage visas) in the account for 3 months prior to and 3 months after application. It's a pain in the ass but we need to play by their rules.

Sadly you are mistaken thinking you can use proof of funds in your country for a year extension off and O-A visa at the immigration office here. There is no "get the Auzzie consulate to notarize a bank document from your bank back in Oz"

The only way you could have done that was to have gotten an affidavit of income from abroad (stat dec) from the Australian consulate here BUT they stopped issuing those back in Dec 2018. :/ Meaning that ship has sailed (a good while ago)

IF you're going to get a year extension inside the country on your O-A entry you bank 800K baht in a thai bank account in your name only for 2 months before you get the extension, the money has to stay in the account for 3 months after the extension is granted and then the balance can't go below 400K baht the rest of the year

OR

IF you want to use monthly income method you would need to transfer in to a thai bank account in your name only a minimum of 65K baht a month, each month, every month for the previous 12 months before you apply for the extension.

Also don't forget to get a year extension off an O-A visa entry you need to be insured by one of the 15 approved thai insurance carriers who issue health insurance on O-A's.