I've tried to check what new plans I would be able to change to via: https://offer.optus.com.au/mbbrpc/ but every time I input my details it says:
"Oops! We're unable to locate your details in our system. Please check you have correctly entered your number or username. If you're a new customer, it may take up to 48 hours after activation for your details to be updated."
I've been with optus for a year now, I'm pretty sure I should be on the system somewhere...
You'd think. But that process has always been a little hinky (I also don't trust it to show all options even when it works)
If you're out of cotract check out the current offerings on the Optus site (the current $41 for 80Gb is pretty good)
Sounds about right, 1 year ago the retail stores used an old system to provision customers to be signed up. That old system is now gone but issues like this still exist. I have the exact same problem as you.
Your best bet is waiting for an Employee to respond to you here, or, contact LiveChat (overseas consultants), Facebook (Australians) or go into a shop if you live close to one.
Just a note the latest Optus App (seems this is where development is focused ATM) update mentioned something about showing possible plans you can upgrade to - might have better luck.