You can't on any post paid account I thought.. Mind it certainly would be a cool idea but of course it'll kill anything that relies on the data i.e. MMS, iMessage. I guess this stuff takes time like the introduction of the $10 addition as apposed to being charge $2 per MB like the old days. You'll be activally notified up to then over your allotted data package by charging $10 for each 1GB chunk.
You may have to activally monitor your usage via the Optus app and if you're transfer at large chunks at a time allow time between each data session so the usage meter can keep up.