Hi Adrian,
To just clear up a bit of confusion on sending items overseas by Australia Post.
Parcels are only charged by weight when sending to overseas countries.
If you look at the Australia Post link that Ivanll gave you, you will see that that standard postage charges are only worked out on the weight of the item.
If a courier is used for shipment such as UPS or Fedex, then weight and cubed size is calculated, and the greater of the two is used to calculate charges.
If you Post Office is cubing your parcel when being sent overseas, then they are not following correct Australia Post procedures and may be charging you a lot more than they should be.
Hope this helps out.
All the best,
Phil