How does Australia Post calculate postage?

For MyPost Business Users, your postage costs will be calculated based on the following:

1. Origin and destination

Where your parcel is being sent from and going to.

2. Packaging type

• Australia Post flat rate packaging† If you send 5kg or under in Australia using flat rate packaging, your postage is calculated based on the packaging size. There are four sizes to choose from - small, medium, large or extra large.

• Own packaging (This includes all packaging not part of the Australia Post flat rate range). We use a parcel's weight and its dimensions to determine how much space it will take up in our vehicles. That means, postage is calculated based on the parcel’s physical or cubic weight, whichever is greater. By entering your parcel’s physical weight and dimensions in your preferred online sending platform, the chargeable weight will be calculated for you.§

3. Delivery speed and extras

Choose a parcel service like regular Parcel Post or Express Post. Select any extras including cover for loss or damage, or signature on delivery.

† You don’t need to cube your parcel when sending an item 5kg or under within Australia in the flat rate packaging.

§ The chargeable weight of the parcel is based on its physical weight or cubic weight equivalent, whichever is greater. A large or irregularly shaped parcel may have a greater cubic weight than its physical weight.

Resource:

https://auspost.com.au/content/dam/auspost_corp/media/documents/calculate-your-postage.pdf 

For eParcel ParcelSend and StarTrack Users, your rates are negotiated per your individual contract; this could be based on physical weight alone, or physical weight/cubic weight, whichever is greater. We recommend referring to your contract to confirm how you are charged for postage.