You need to contact Optus Live chat to ask what your specific contract entails. They differ a lot.
All require you to pay any outstanding phone purchase amount
Some are subsidising the phone amount and that stops when you leave (so the phone amount goes up)
Some allow you to stop the call portion of the plan immeadiately with no penalty
Others have a penalty (i.e. $300) that is pro-rated down as needed (so 4 months of 12 month contract = 1/3 = $100)