Do I need an onward travel ticket to re-enter Thailand after visiting Vietnam on a tourist visa?

I am in Thailand on a 60 day tourist visa. I have received a 30 day extension till 14 May. I arrived by air from Australia. I am an Australian citizen. I plan to travel to Vietnam by air for up to 3 months around 13 May. For my original 60 day tourist visa I had a return flight to Australia on 13 April. This will now need to be changed or cancelled. I am uncertain what I will do following my time in Vietnam and would like to keep my travel options open including any return dates to Australia. I may wish to return to Thailand for 30 days. To enter Thailand again would I require an onward travel ticket either by land or air? Are there any other requirements that I should be aware of? I have only previously had one 60 day tourist visa to Thailand in June to July 2018 without a 30 day extension.