I had the exact same problem with Optus when I moved back to Brisbane. It took me approximately 3 years of fighting with the credit reporting people and Optus.
I had Optus cable in Sydney, which was awesome. When I moved back to Brisbane, I checked with Optus whether my new address could take their cable, as I wanted to keep it before I signed the lease. They say yes, no problems. I organise with them to come and connect the cable up the first week I'm back in Brisbane, and they don't turn up. No calls, no notes, just nothing.
I ring them and they tell me that the address doesn't support Optus cable despite them telling me it did. So after much gnashing of teeth and facepalming, I cancel my account. They wanted to charge me an exit fee, but I talked them out of it.
I get a letter from Optus saying all done and dusted. Account closed. at the same time as a bill asking for a connection fee and a months access. I ring them and explain he situation, they say "very sorry, we'll fix it, you can ignore the bill."
I received another 18 (. ) bills and numerous letters over the next 18 months from them, all of them getting nastier
and nastier. Every time, I ring them up, they say they'll fix it. I emailed, wrote snail mail letters explaining they were charging me for a service that they couldn't physically deliver, and one that I'm not receiving. I sent them a copy of the letter saying the account is closed. It wouldn't stop. At this point I give up and just ignore them. I then moved, and I never heard from Optus again.
Fast forward a couple of years, and I go to buy a car. They won't give me a loan because apparently I owe Optus around $3000. I ended up having to pay another $6k deposit on the car in order for the bank to give me the loan so I can get the car.
Fast forward another 2 years or so, and finally I get it cleared. In that whole time I couldn't get a phone on a plan or anything, and had a s*** loan on the car.
I enquired about suing them, but apparently it was going to cost a fortune, and there were no guarantees I'd get any compensation at all.
Optus are complete incompetent c****ucking arse reaming f****ticks who couldn't organise a f*** in a brothel, a beer in a pub or a sausage at a BBQ.