Is it legal for the operator to ask my specific income once I pay the overdue bill? She asked how much my fortnightly income would be after I pay that bill. I have worked in collections before and we never asked this. I found the question intrusive and none of her business.
Its legal to ask anything. Its also generally legal to politely decline to answer.
I would say that you have entered into (and are entering into) a credit arrangement. The law is that creditors (Optus) must ensure that such arrangements are within your capacity to pay. Your anticipated income (now and in the near future) isn't unreasonable IMO.