04-09-2015 07:08 - edited 04-09-2015 07:16
You should have got your phone unlocked before you moved to Australia it would have been much easier
You need to contact customer services ....from abroad +44 3448090202 Free
Also have a read through this unlocking guide
Edited to add. The phone wont automatically be unlocked just because you have paid it off....hence the need to contact CS
on 04-09-2015 08:54
No need to pay off the phone anymore on the 6 as O2 have lifted that restriction now.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)