Some providers might not provide the coverage you require. Also, make certain your unlocked GSM handset contains the necessary frequency bands for the providers/region you intend on using the service in. It's often overlooked, btw. If you post the make/model of your handset we could double check compatibility. Generally, GSM is on 900/1800 in Oz. Many bare bones GSM handsets sold in the US are still 850/1900, for North America. More info here by country-
It may also be easier to simply add international roaming to your U.S. based cellular account for a month, especially if you do not anticipate making many calls for lengthy periods of time. The prepaid SIM cards you purchase in Oz will be cheaper for placing local calls while in Australia, but not so much for dialing internationally.
Before you commit to any service, you might also consider migrating to a cheaper solution such as Talktatone/Google Voice (free calling over wifi), or even Skype (with international calling credit you can purchase). These would require a PC or smartphone to set up. For minimal calling, it might be preferable depending on your preferences.
We are traveling to Australia next year and need to get a SIM card for calling back to the US occasionally to talk with my 90 year old mother and her caretaker. We have an unlocked GSM SIM card phone. My main concern is with companies who market the SIM cards with talk time. Which are reputable, and which are not so. Any advice will be appreciated. Thank you.