Rollover is not a general prepaid feature. Each provider states which recharges carry unused data forward, and the amount you recharge is often what decides whether rollover applies at all.
Rollover rules as published
- Boost states unused data rolls into your next recharge when you recharge $28, $39, $49, $59 or $74 before expiry.
- The Optus prepaid data only plan states you can roll unused data up to 200GB when you recharge before expiry.
- Telstra states you can save up to 200GB of unused data when you recharge $10 or more before expiry.
What the condition actually is
Every one of those rules has the same shape: you must recharge before the expiry date. Miss the date and the rollover condition is not met. The three providers express it differently, which is why reading your own provider terms matters more than a general rule.
Note the caps. Optus and Telstra both cap the amount you can bank, so rollover is a cushion against a bad month rather than an unlimited stash.
What we could not verify. Where a provider has not published a figure we mark it unknown rather than estimate it. Re-read the provider page linked below on the day you decide.