Long story short, a SIM only plan is a postpaid plan which basically gives you a line of credit (you can use as much as you like and then pay later). In Australia, for you to be legally able to enter an agreement like this, you must be 18. No way around it unfortunately unless you put it into someone else's name (like your parents).
Your alternative is pre-paid which offer pretty competitive plans these days, at least until you're 18
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I do not represent Optus. The views, opinions and advice expressed in my posts are my own
