Optus Live Chat isn't able to answer this question apparently ....
Wen in My Account, where is the function to set up or edit data usage alerts?
They are automatic on all post paid mobile plans.
Set your data counter / limit on your phone and you should get a SMS around the point you hit 50% of your monthly usage.
Note, you can't edit the warnings: 50%, 85% and 100% and each 1Gb top up thereafter.