Optus keeps billing me even though my plan should have ended?

I got an iPhone 6 in september 2014 and got the $60/m plan and the guy told me it's only going to be 12 months (which is what I asked for) but they're still billing me? Was I supposed to cancel it? Why are they still billing me?

Yes you should have cancelled, when the contract ends, the plan continues if you don't make any changes, Optus are a bunch of snakes, but that is normal among all providers

It wasn't a 12 month contract, it was a 12 month minimum contract. That means you're free to notify your provider that you 'd like to end your contract once the minimum period has expired and many contracts state 30 days notice. So for a 12 month minimum you should have notified the provider 30 days before the end of the minimum contract date if you'd wanted it to end dead on 12 months.

Minimum contracts rather than fixed term contracts are completely standard for all telecoms, Internet and subscription TV services along with many other services with recurrent billing.