Is there the ability to set up Direct Debit with a maximum limit? I'd like to have direct debit set up to automatically pay my account, but don't want to give Optus unlimited ability to drain my bill payment bank account. I'd like to be able to limit it to the standard amount of my bill, and then if there's a larger bill I'll pay the extra manually. Thanks,
There is no capacity to do this sorry.
Not that you should get an email bill a week or two before the direct debit is made so you can review it ahead of time.
You can also cancel a direct debit any time unilaterally via your bank (although you can also cancel it in MYACCOUNT to anytime)
I agree it is unfortunate that Optus have so many areas where excessive charges can occur (Premium SMS (until recently), Global Roaming, Data Top Ups, etc.) with few tools to contain these costs. But if you do incur these costs then Optus is legally owed the money, so stopping a direct debit will not stop the ultimate payment of the amounts.
FWIW if you best want to protect yourself then a prepaid card has no chance of losing more than the amount on it.