I have accidentally discovered a way of using up lots of prepay credit.

Some phones (eg the VF Smart Ultra) have a default option to send texts by MMS if they are sent to a group of users - presumably this allows richer features like reply to all.

The catch is, 2degrees doesn't include MMS in the value bundles, so every time you send a text to a group of 8 recipients, that's $4 off your balance.  We only realised this when my wife realised all her credit had disappeared after sending a few organisational messages for the kid's football team.

So, if you're using group messages, I recommedn checking in the messaging app advanced options that "send by MMS" is not selected