Also - seriously - look at Sendle (
www.sendle.com.au) - the price I got for 3kg, 50x45x25cm was
$28.95 door to door. They do international too.
I'll check them out I have used EGo Couriers online for years they are part of the Hunter group and use sub-contractors throughout Australia.
On some routes they are much cheaper than AusPost too but the insurance costs are far higher and without the guarantee you get from AusPost. Some semi rural or rural locations can be a lot dearer than Australia Post.locations
I have shipped and had picked up for me car and m/cycle engines and panels, sets of wheel/tyres etc around the country for years. Large bulky items are often quite llght for their size and Couriers charge by volume so if you have 'sweet' PO staff they often just charge the actual weight rather than by volume which can nearly halve the cost on occasion

. Unfortunately the AusPost parcel charge calculators online FORCE you to provide dimensions for a volume charge calculation as does my local corner shop PO agency who often have no clue how to process anything.
If using AusPost I ALWAYS go to my local actual PO (luckily only a few KM away), and they love me.

AusPost will even ship full R3 TORs exhaust systems 17KG packed and just within the 105cm length and 140 cm girth, 20KG max rules. It makes my day when they use weight and not volume to charge me then.
Similarly screens are probably a lot cheaper to send safely via AusPost.
The big advantage of getting large items that interstate sellers on eBay etc only list for 'PICkUP ONLY' but they will often pack for a home pickup by courier when requested if you arrange and pay for it all online and explain they only have to print off the shipping label and leave it at the door or carport for pickup without them needing to be home.
Courier Company Australia: Interstate Parcel & Courier Delivery Services