This is the question that separates a prepaid plan from a postpaid contract, and the published answer is unusually clean. There is no early termination charge. There is an unused balance, and it disappears with the service.
The published position
The TPG summary states that TPG Mobile Services are supplied on a rolling month to month basis, that customers are permitted to terminate the acquisition of the Service at any time, but that you will forfeit any prepayments already made. The early termination charge field on all four SIM only plans reads not applicable. So the cost of leaving is not a penalty; it is the money you already paid for a service you stop using.
What makes prepayments exist on a monthly service
Recurring charges are payable monthly in advance, 7 days before the start of the next billing cycle, and you can make additional prepayment if you want to make excluded calls or usage that will incur excess charges. That last clause is where the balance builds up: extra prepayments for international calls or excess data are exactly the money a terminating customer forfeits.
The decision rule
If you are likely to exceed your included data or make international calls, prepay deliberately and withdraw deliberately. If you are not, pay monthly and there is nothing at risk. The plans on this site are compared on the ongoing monthly figure rather than the deposit, because that is the number you will actually live with.
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.