Optus do not provide an option for post paid plans for this. Be happy its only $10 as Optus T&C state they'll keep toping you up 150 times before they will stop providing data (you do the maths!). Couple that with dodgy updates on usage and you've got yourself a money making machine (or rather Optus do)
But there are somethings you can do to protect yourself from Optus
The prepaid option is definitely worth investigating as it is infallible. It also has rollover data which post paid doesn't. Otherwise plans are largely similar in pricing and features.