There are a heap of third party options that allow you to monitor your call and data usage in real time, but today Optus has moved to crush them like this thing crushes cars thanks to their free app.
The 'My Account' app is available from the app store now as a free download. It will let post-paid customers view their unbilled usage for eligible iPhone plans, view their last four bills, or pay their bill by credit card. Pre paid customers also get the option to view their balance, view up to 100 of their most recent transactions or recharge their account via credit card or recharge voucher.
The app also doesn't count towards your data cap when you use it on the Optus network (as opposed to overseas), which is something the other 3rd party options can't offer.