from the link on the AP user agreement

http://auspost.com.au/personal/parcel-dimensions.html

Irregular shapes

To find out the length of an irregular shaped parcel measure between the two furthest points on the surface of the article.

This length should not be more than 105cm.

To find out the girth of the item, measure around the widest part of the parcel at right angles to its length - this measurement should not exceed 140cm.

Australia Post reserves the right to apply cubic charges to a parcel weighing less than 1kg and to parcels that are irregular or cylindrical in shape