http://old.vodacom.co.za/services/vodacom_broadband/carryover.jsp
DATA CONTRACT & MONTH-TO-MONTH SPECIFIC RULES
Unused allocated data (MB) will be carried over up to the last calendar day of the next month.
Example 1:
A customer subscribes to a My Meg 500 24-month contract on 1 May and is allocated 500MB to use up to 30 June.
On 31 May, the customer has used 250MB of the allocation and on 1 June is granted 250MB carry-over (carry-over equals 500MB allocation in May less 250MB usage in May).
On 1 June, the customer is allocated 500MB to use up to 31 July; the 250MB carry-over from May is added to use up to 30 June.
On 30 June, the customer has used 250MB which is less than the monthly 500MB allocation and forfeits the 250MB carry-over from May; the customer however has 250MB carry-over to July (carry-over equals 500MB allocation in June less 250MB usage in June).
The new data bundle allocated at the beginning of the month will be used first, before the carry-over data bundle.
Example 2:
A customer subscribes to a My Meg 500 24-month contract on 1 May and is allocated 500MB to use up to 30 June.
On 31 May, the customer has used 250MB of the allocation and is granted 250MB carry-over (carry-over equals 500MB allocation in May less 250MB usage in May).
On 1 June, the customer is allocated 500MB to use up to 31 July; the 250MB carry-over from May is added to use up to 30 June.
On 30 June, the customer has used 700MB; the total monthly allocation for June is used and 200MB of the 250MB carry-over is used; thus the customer forfeits 50MB left over from the May carry-over.
So.. as you can see its a little more complex than "You can carry over 1 month of unused data."