That is an unacceptable down time. What NBN technology do you have? FTTN. HFC?
With that much down time there is obviously an issue. Optus is responsible for the service (they can't just say its NBNs problem). You need to request Optus to send out an NBN technician to investigate.
Are you keeping a rough log of down times? Optus also automatically logs all disconnections to your modem so they should also be able to confirm the issue on their end.