I used to receive a PDF invoice attached to an email. Now Optus doesn't bother to even attach an invoice - just sends a demand for money. I spent half an hour on the phone to Optus last week and was told that I would now receive a PDF invoice, but this has failed at the first opportunity.
It's a pretty lazy move. It's inconvenient since the bill needs to be split between business and personal and, as a bookkeeper, I don't pay bills without an invoice. If Optus wants to make it as easy as possible for me to pay my bill - email me the invoice like you used to. Otherwise I will find a provider who will.
Odd, I and many others still get a PDF invoice.
Have you changed e-mail address or anything? Maybe it needs to be verified on the Optus end again?