Watch any of the 6:30pm current affairs shows and you'll see that people getting caught out by expensive smartphone bills - often called bill shock - is quite common.
Provided you stay in Australia and don't do any international roaming where the bills skyrocket, it's actually pretty easy to keep track of your data usage through Android apps.
Unfortunately, not all phones come with an app for this built in, as it generally depends on the phone provider. While more recent versions of Android - since version 4.0 do let you track data, older phones running Android 2.3 don't.
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However, most of the major Australian telcos have apps on the Android Market that will help you monitor your usage.
Know your limits
The key with all of these is to make sure you know what your monthly data limit is.
Because the telcos offer different data limits on different plans, it's up to you to know your limit and stick within it to avoid any nasty surprises at the end of the month.
Unfortunately, not all telcos have official apps and those that do aren't always as good as some of the user-created versions.