All calls from an Optus mobile phone are free to Optus (even from overseas)
You are with the NBN now (who are responsible for the in ground connection to your home). However Optus are resonsible for getting the NBN off their backside if they don't already know.
What street and postcode are you near?
What NBN Technology are you using (Wireless, FTTN?)
I suspect water in the cables is a big problem around the nation right now (Thanks Tony Abbott for your FTTN technology fail)
BTW I'm not from Optus. You will need to get onto Optus directly (Live Chat via the Optus App on your phone?) and report your outage. If others are also on it then I suspect NBNCo already know your area has an issue.