If you have unlocked mobile, a Boost sim is available in Coles - look on website and retailers stores for availability. Sim can be activated online and it doesn't take long to do so. Choose amount you want to recharge on sim and go from there. Very simple.
You will need wifi turned off and data turned on to check your balance and recharge . Hope this helps.
Don't forget to book accommodation etc before you arrive. Have a great holiday and safe travel.
If you have certified copies of the passport / ETA then that would be good I would assume. I am not 100% sure if they aren't certified...
For locals our system isn't overly painful because we can just activate no issues but we do have pretty strict ID rules for mobile services. It is meant to help control the movie stereotypical 'burner SIM' sort things.
I have heard of online activation working with international passports for prepaid so it might be worth a try if you have people who can get one for you.