No, the law is that the GST is charged on the usage, not the topup.
This is why they say the GST receipt is based on usage, because that is how the tax law is written.
So how does that work when you get the topup at a supermarket and it has a reciept?
Surely 2 places cant make an invoice on the same product sale?
Indeed. Surely if you were to claim GST on a receipt from the supermarket, and then subsequently from your provider that would be fraud. I'm surprised they allow such a situation to occur.