The reason why banks have to wait 28 days for automatic deduction in Australian training courses is the bank processing time and payment cycle.

1. Bank processing time: It may take some time for a bank to process debit card orders, depending on the efficiency and system of the bank.

2. Payment cycle: Some banks may stipulate that the payment should be received within the specified time limit, not the actual deduction date. This will take some time to deal with.