Three weeks is not a quick turnaround. In fact with AusChecks current performance it is a little below average.
Any card that is quickly processed should not be an expectation of how others will go. Even per individual, the times can vary greatly from one renewal to the next. Best advice is, get your application in by the first week of the month of expiry or even a few days before the end of the previous month. If you put it in too soon, you run the risk of the clearance coming through quickly and losing a month.