Optus charge you 1 month in advance for almost all of their services. Chances are, the bill you've got was generated before your service was cancelled. The system will automatically trigger a refund once the next bill is issued, they just can't issue an interim bill.
You'll need to contact Optus directly and ask about the bill. Possibly the cancellation hasn't rippled through the system properly. Unfortunately it can possibly take several attempts to get it to stick despite assurances otherwise. You should get a refund on any incorrectly billed amounts though.