The 2012-2013 financial year saw a big increase in the tax-free threshold -- the amount of money you have to earn before you pay tax -- from $6000 to $18,200. If your total income fell under that amount, which could happen with part-time work, you might not need to lodge a return at all.
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The key issue here is whether or not tax was taken out of your wages. If it was, then you'll need to lodge a return even if your income fell underneath the threshold. The good news is that you'll definitely get a refund in that case.
If you're working multiple part-time jobs, chances are you will have only claimed the tax-free threshold with one of them, so you'll need to lodge a return and include both the jobs which took tax out and those that didn't.
Final point: while you may not have to lodge a tax return if your income fell below that level and no tax was taken out, you should still submit a non-lodgement advice form.