Two different things get called a lock in a mobile plan. One is the contract term: how long you are committed to paying. The other is the device lock: whether the SIM will work in another phone or on another network. A no lock-in prepaid plan is about the first and usually has nothing to do with the second.
Contract term
Australian prepaid plans have no contract term at all in the postpaid sense. The amaysim catalogue states no lock-in contract on its rows, and the TPG Mobile SIM only service is supplied on a rolling month to month basis that can be terminated at any time, forfeiting prepayments already made. So on both counts the commitment is short; what you lose by leaving early is unused credit, not a penalty.
Device and SIM portability
The other lock is a different matter and is priced separately. The Boost summary sets out an unlocking fee that applies to a device: $80 within the first six months, $25 after six months, and $0 after 2 years. That is the cost of moving a device bought on a plan, and it is separate from any contract term on the service itself.
Keeping your number
Prepaid makes the number question explicit. To keep your number you must keep your service recharged or transfer it to another provider. There is no contract holding the number for you; the recharge history does. Let the service lapse for long enough and the number is at risk, which is the real difference from a postpaid number that stays with the account.
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.