Australia SIM recommendation?

[inquisitor]

Amaysim has very good data as well as voice tariffs, especially for international calls:
amaysim SIM card - One pure SIM with simple, low mobile rates

But make sure that your handsets supports 3G on 900 and 2100 MHz if you want fast data since these are the frequencies the underlying MNO, Optus, uses.
If your handset supports only American 3G bands (850 and 1900 MHz) you're limited to the 3G networks of Telstra and Vodafone, albeit the latter's 3G coverage on 850MHz is pretty limited (see "Network Zones" tab at Coverage checker - Vodafone Australia).

For an overview of Australian networks and MVNOs including their frequency bands check out Australian Mobile Providers - Carriers & MVNOs | IDD