Are you on pre-paid or contract? Last time I checked, it worked the way you described on MTN pre-paid. I don't know about contract though.I am surprised because my understanding was that on MTN the oldest bundle is used before the newest. I also at times when travelling abroad had three bundles accumulated on my iPhone.
There's a long-running thread about this:
http://mybroadband.co.za/vb/showthr...equest-to-recosider-your-data-rollover-policy
iBurst have a very simple rollover policy:
Unused data will carry over to the following month up until a maximum of three times your monthly cap.