So sorry for the hassle @Jamesy. To get Prepaid recharge receipts you'll have to refer to your Credit card/bank statements this is because Optus doesn't store your receipt details for online recharges. But if you click on usage details in My account you may be able to see your most recent recharge.
Everything listed or any payments/recharges made are inclusive of GST. Hope this helps!
All official selling services around Australia (and western countries) are obliged by law to keep track and provide official invoices to their clients for any service or product that they sell. This allows the Financial Department to keep track of the money, to avoid any kind of tax fraud. What is happening with OPTUS with this issue is illegal, or should I say, in the border between legal and illegal. This allow OPTUS to save millions of dollars because they "delete" the selling history, and with that, clients cannot use invoices and the Financial Dep. cannot cross results in the end of Financial Year.
NOT PROVIDING INVOICES IS ILLEGAL.