Read all about it here:

https://www.abc.net.au/news/2024-11-27/australia-post-ai-miscalculates-parcel-dimensions/104628682

Australia Post have upped their game last few months with new AI generated algorithm which images your parcels and compares the dimensions against the postage paid dimensions.  An automated process to weed out underpaid items.

If you use MyPost Business, you will receive an email and notification of under payment.  Early on we got caught on 2 discrepancies, but as we photograph every parcel prior to dispatch, were able to provide evidence and reverse the payment.

Support staff were able to pull up the data they recorded and possibly an image to compare to our own and admitted it was a way off recording by their equipment and said it was 'early days' in the technology which they are still perfecting.

If you use eBay Postage, Australia Post will bill back eBay then you for under payment.  Much easier to dispute underpayment with Australia Post using MyPost than via eBay.

AP did not charge you the initial postage, it was charged to you by ebay and AP will recognize ebay as the purchaser of your particular label, hence why AP have sent the demand for additional fees to ebay and not direct to you.

In transit, your item will pass through AP automated measuring and weight devices, which are normally very precise.  If any discrepancy is detected additional fees may apply.

The measurements you have provided would seem appropriate for a flat  TV style box, what measurements have you recorded for the article.  Note a 5mm error can be sufficient to incur additional fees.  Do you have photographic evidence of the parcel weight and dimensions which includes measuring devices in situ.