@Valerie *****
Your on a tourist visa or visa exemption with 30 days extension. You can apply for 90 days Non O visa in country first on a TM.86 or TM.87 form. After 90 days you can apply for 12 months extension of stay. The fee is 2000 thb.
As you’re from AUS, your embassy doesn’t offer to provide an affidavit letter to confirm your income/pension for the Non O or extension of stay. Therefore the first thing you’ll have to do now is to open a bank account and put 800K thb for fulfil financial requirements for a Non O. It’s not easy to open a bank account in Thailand now without a Long Stay visa, so you might need to get an agent to help you with this first step.