Tested, this *136*9# works on PREPAID as well.
FYI, for prepaid user, you can check your internet usage since long time ago. In *100# menu select 7 Internet dan Tetapan > 1 Mobile Internet (MI) > 8 Check Status.

I tried, and tested it works for Prepaid users as well.
Another alternative for Prepaid users to check usage:
1) Dial #100*
2) Select 7 Internet dan Tetapan
3) Select 1 Mobile Internet (MI)
4) Select 8 Check Status then you can see your current internet usage

The numbering order may change from time to time since Maxis always add in/remove options from their #100* menu.

You can also subscribe to data plan.
In *100# menu > 7 Internet dan Tetapan > 1 Mobile Internet (MI) there is a lists of available subscriptions.
If your data plan is expired, yes they will charge on your prepaid credit based on 10 cents/10 KBytes.
That's why your credit balance will gone in seconds.

This post has been edited by huiqing88: Sep 27 2011, 11:13 AM

Be careful later bill shock