If you apply for an OA Multiple entry visa, you can show AUD 54,545 in an Australian account (which is a lot more than 800K THB). For a NON O (retirement) visa obtained in Oz, you need to show AUD 2000. If you will obtain a Non O visa based on being over 50, in Thailand and then a 1 year extension of stay, you will need to transfer 800k into a Thai bank account.
It also does seem to make a difference if there is time spent out of the country in between tourist visas or visa exemptions. Visa runs, as you mentioned, where you jump a border, get a visa, and come straight back, or a border run with an exemption, is a good way to attract the wrong kind of attention.
Why are you just thinking of this now? You can renew a passport well in advance if you plan it. I had the same issue with mine. Renewed it 9 months before expiry and got the extension stamp transferred.