Just want to verify, as I don't think it is a large base where foreigners enter Australia...
Are there shops in the Adelaide airport that sell SIM cards?

Last year at Melbourne, I bought an Optus SIM card and it looks like they still have a similar deal:
https://www.optus.com.au/prepaid/tra...E&gclsrc=aw.ds

On this trip I'll just be traveling around Adelaide, Melbourne, and Brisbane (possibly Surfer's Paradise). I won't be going too far out of major cities, so I think that service should be fine for me, if I can find it at ADL.

Otherwise, I'm flying ADL-MEL just after I arrive, so I could pick one up at MEL, or in Melbourne the following day. Having a SIM card would make it easier for a friend to pick me up from MEL (unless there is a designated pick-up spot at MEL).