For the benefit of anyone else who has the fuel reading issue in an AU wagon, here is an update:
As the car needed a few things done (hubs, rotors pads, tensioner bolt seal, tranny service etc), I decided to get my mechanic to do the whole lot including dropping the tank and installing a new sender.
Now Ford no longer sell the fuel sender by itself (but do sell the pump and sender assembly for the sum of $370
). But you can buy the assembly aftermarket for around $170, it is made by Delphi or VDO who made those things for Ford anyway so as far as I'm concerned they are as good as genuine.
But I didn't have to buy the whole assembly, on ebay I found a NOS genuine fuel sender which suits an AU UTE and got it for $40, my mechanic then swapped the float arm (the only difference between a wagon sender and a Ute one as the module is the same) from the wagon and put it on the new sender (have to be very careful when removing the arm not to break the plastic clips), and installed the lot in the car. Happy to say it now reads correctly and works a treat.
Also worth knowing that the Commodore fuel senders of the same era are identical to the Ford ones and are made by the same mob (again Delphi or VDO) with the only difference being the float arm again. This is handy as Commodore senders are apparently widely available on their own and also cheapish.
Hope all this helps someone.