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As an international student who is leaving its something you can ignore. All you will get is a bad mark on your (Australian) credit record and in Australia after 7 years they disappear anyway. An outstanding debt also has no impact on visas or travel in anyway.
That said if you are cancelling your plan then don't use any data. You bill suggests you are still using your phone and have used 100Gb of data over your limit. Just stop using your phone.
To cancel just write to : PO Box 306 Salisbury South 5106 SA