Log into your Optus 'My Account' page, then go to 'Dashboard' and below 'Bills', your service/s (phone number) should be listed.
If you have only one service, it will already be open.
If more than one, select the one youn want
In there you should find a 'Your Usage' heading, displaying the info you want.
Be aware that the info displayed can be up to 48 hours behind what you usage is at NOW.
Hope that helps.