I have never had a return ticket, nor been asked for one, when flying from Australia to Thailand with a current visa in my passport. The only time I have ever been asked about onward travel is when I got a 30-day visa exempt stamp flying in from Seoul.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.