How does the Australian multi-entry 6-month tourist visa work in Thailand regarding extensions and visa runs?

Yes, you are correct. With your 6-month multi-entry tourist visa, you receive a 2-month stamp on arrival in Thailand. After the initial 2 months, you can extend your stay for an additional 30 days. If you then perform a visa run, you can re-enter for another 2 months and apply for another 30-day extension. The visa validity period, however, begins from the date of issue, so it's advisable to enter Thailand as soon as possible to maximize your stay.