As the first writeup is referring to the Australian
banking network, I can shed some light on why it takes the amount of time that it does for a cheque to clear in Australia.
The delay essentially gives the banks some time to intercept fraudulent cheques. After the cheque is deposited and processed overnight, it is returned to the originating bank. It is during that day that the signature and the date are checked, and the cheques are also inspected for any unauthorised alterations.
If the cheque is found to be fraudulent or non-transferable in some way, the originating bank will contact the processing bank and let them know the cheque has been dishonoured. They then forward this notification by mail to the processing bank. The processing bank then advise their customer of the dishonour.
Note: only cheques over the value of $20,000 are actually inspected by the banks. The reason cheques of smaller value also take the same length of time is primarily for consistency's sake, and also as a time buffer in the event that the bank is advised of a fraud for a cheque of a smaller value.