Are you in QLD mate? I'm an electrical contractor, i can help you out.
In QLD it is free to have the aerial line to your house upgraded, as long as you have had an electrical contractor upgrade from the "point of attachement" back to your switchboard.
If your switchboard is an old piece of junk, it would cost you between $1200-1500 to have it completely upgraded. Thats new everything, box, safety switches, new 16mm mains cable, Everything! Once thats completed energex will come out and upgrade the aerial mains for free.
If your switchboard is in good condition and its just the mains box that needs to be changed. It should cost under $500 to get someone out to replace that, then energex will replace the aerial mains.
hope this helps