If you have not downloaded or printed the label when first generated, you can request a reprint by simply clicking the Print/Pay button again. This will not create a second label, or a second charge.
Already Fulfilled and Updated Store Labels?
If you have already fulfilled the order, you need to log in to your MyPost Business account and go to the History tab where you can choose the batch of labels to reprint.
(https://auspost.com.au/mypost-business/shipping-and-tracking/history)
Note: Australia Post only allows labels to be reprinted for 48 hours after they've been generated.