Nothing about a prepaid plan is instant or anonymous. Before the first call or the first KB of data there are three requirements, and each of them has bitten someone.
One: a compatible handset
The TPG summary states you must have a compatible mobile handset to be able to use the service, and it is explicit that acquiring a handset from the provider is not a requirement. The Boost summary goes further on what compatibility means: an LTE band 3 (1800MHz) and band 28 (700MHz) capable device is required, and an incompatible device may be blocked and may not have access to voice or data, or be able to call emergency services.
Two: identity verification
Australian prepaid activation requires identification, and every provider publishes a compliance section requiring it. We hold those sections in archived form for the plans we cover, and we do not reproduce the personal identity requirements on this site, because they are the providers to state to you directly and they change.
Three: an activated, recharged service
Prepaid credit does not arrive by itself. The service activates on a recharge, the term starts on that date, and the usage alerts begin from there. That is why a prepaid plan cannot be bought as a present and used a week later; the clock starts when the money does.
The practical order
Check band compatibility on your phone model, complete identity verification, then buy the shortest cycle that covers your first fortnight. If the coverage or the handset is marginal, a cheap first cycle is the cheapest possible test, and every provider here sells one.
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.