NonprayingMantis: just be VERY careful with Optus (and Amaysim whihc is an Optus MVNO annd uses the same billing method) as they round each session up to the nearest 10MB - meaning if you do lots of short sessions (checking email, tweeting etc) your data usage will seem to skyrocket
Amaysim round to the nearest 1MB (not 10MB). But at $19.90 for 2.5GB, it doesn't really worry me.
Compared to the pricing of WiFi at the apartments we're staying at (or the prices back in NZ), it's very reasonable and will last us for the holiday.