No OPTUS won't customize.
Their reply was:
Meanwhile, please know that the automatic data top up on your $50 My Plan Plus 12-month Sim mobile plan is built-in on the plan and we cannot remove or modify it. We came up and pioneered this mobile plan feature as we deemed that this is more bill and customer-friendly compared to the old plans that we had from which our customers are charged $0.25 per MB of data.
I already follow most most of the hints in the link Jordan supplied.
Data turned off for apps no auto launch of video on Facebook. No auto updates etc
One suggestion to me from chat was to turn off data once I get the 85%usage warning (but they don't always get sent).
I just dont understand why the app updates regularly for most of my statement month but is always on a go slow on the last day as I try to wring all I can out of what I have paid for but not go over. I usually do well and have 200-300 mb unused. But it bites when I have over a gig unused.
Mostly the app updates 4-6 hourly never on the last day. optus warn that the app usage is not in real time and can take 48 hours to update.