do you know who I need to write to regarding the weeks that i did not have any internet. I've checked the invoice and they have still charged me for the internet package despite my internet being down for weeks.
Customer Relations Group PO Box 306 SALISBURY SOUTH SA 5106
Note that there's no specific requirement to refund customers for down time. Residential plans are expected to fail on occasion. In your situation Optus would usually have supplied a temporary 4G wireless option (if usable).
If you've had an outage fora few weeks the easiest thing is to just contact billing directly (LiveChat, Optus Facebook, Mobile App, etc.) and they should give you some credit on the spot.