Change your postage option to 'Freight - large and bulky items'. Note it in your listing that prospective buyers are to ask for a postage cost to their postcode. Learn how to use the Australia Post Postage Calculator - it is not hard, even I can do it now. For a few times I took packaged articles to the PO for confirmation that my calculations were correct. After that, no problems at all. It is a lot simpler to do postage costs this way, as the costs do vary so much from state to state.
The other bonus is that it prevents buyers from paying for an item (WITHOUT including postage costs), until you have sent an invoice to them.
At least this way, because you have quoted to a specific postcode, and the buyer places their bid, there is no confusion as to how much it is going to cost them to get the item.
