Can an Australian retiree obtain a Non-O visa and split residence between Thailand and Australia?

Yes, as an Australian citizen over 50, you can obtain a Non-O visa for retirement purposes, valid for 90 days upon entry. To extend your stay beyond the initial period, you need to meet financial requirements, such as having 800,000 THB in a Thai bank. Once you have the extension, a re-entry permit allows you to spend 6 months in Thailand and 6 months in Australia, provided the timing for extensions is managed correctly. Alternatively, you could consider the METV, which allows a stay of 60 days at a time for up to 6 months but does not offer the same benefits as a Non-O visa.