Last two months my data exceeds by 2-3gb per day in last 5 days of month.
I received email warning 85%,90%,100% All at once. Service does not slow down. Turn modem off. I still get charged over $100 extra. Optus there is a problem.
Okay, you need to be aware if you do not have "slow down" enabled on your service and still are on a plan with $10/1Gb data extra charge, then with the way billing (for all providers) is managed, its delayed by in some cases up to 48 hours.
So if you know you are getting close, be careful.
The problem is with your usage rather than with Optus themselves, if you've got $100 over, this means you've used 10GB not 2-3GB. Because $100 / $10 = 10 x 1GB Data blocks.