Q: Using US iPad in Australia

a prepaid microSim

or a subscription with an Australian carrier and their microsim

You can just purchase a SIM in Australia and setup service with a local cellular ISP while you're there OR you can setup international roaming with AT&T.

Take a look at this Apple document for additional information -> iPhone and iPad: Using cellular networks while in another country

Look in your iPad manual.

There's a SIM card slot on the side of the iPad Wi-Fi + 3G models (Your inital post shows a iPad 3G, which I assume is a iPad Wi-Fi + 3G model). Look for a SIM slot on the side of the iPad, like the pics below.

Pics from -> iPhone and iPad: How to remove the SIM card

If you have a 3G iPad, you can a microsim from an Australian carrier to connect via 3G.

If you have a wifi only iPad or don't want to use 3G, you can connect via wifi, wherever you can connect to wifi.

And if there is cell service, but no Wi-Fi, you can see if the cellular service provider has something like a MiFi.

Here's one provider with a portable hotspot, which I believe is like a MiFi -> http://www.internode.on.net/residential/wireless_broadband/nodemobile_data/hardw are/portable_hotspot/