It is one of the most requested items by Optus Customers but Optus have decided not to provide that option for post paid contracts. You have two main options:
1) Android Phones have a built in data usage monitor. They can be set to warn and then cut data at a specific level. Not perfect but setting it would ensure you remain under your usage limit.
2) Switch to PrePaid. Optus have decided that they will cut your data usage the second you go 1kb over the amount of data you have purchased (funny that). Prepaid also offer rollover data on some plans so that's also a plus.