For the best chance of having an in-transit item successfully recalled, it needs to be recalled via the Track & Trace section on the AusPost Business Support Portal. This will ensure it can't be scanned on-board for delivery.
To recall an item that is in transit but hasn't yet been on-boarded for delivery:
Login to the Business Support Portal: https://auspostbusiness.force.com/bsp/bsplogin
Click the Track & Trace button.
Search for the item using the tracking #.
Click the Recall button.