I had some 1/2" cone or dowel washers sitting from when I owned my M135, sold when having a sudden fit of responsibility a while back. Those had been sold under the Papco brand from H Paulin & Co who are a Canadian Hardware distributor. See page 5 of their Wheel Parts catalog
http://www.hpaulin.com/Catalogue/WP509.pdf Their part number for the 1/2" version is 565-498 (7/16 is 565-497). Both of those numbers are for the split cone style. I bought my 7/16 dowels from Carquest but when I went to reorder, they seemed to be having a hard time getting more.
Then I went round the corner to Malmberg Truck service. The counter man's reaction was "how many do you need". The brand he had was Euclid, "An ArvinMeritor Brand" see their web listing for Wheel attaching parts
http://www.meritorhvs.com/MeritorHVS_Documents/UD60.pdf page 31 for their listing of dowell washers, I've only seen the E5737 wedges for 7/16 studs, these were not split like my originals, but this might be a good thing in some circumstances. The Euclid catalog also has quite a variety of spindle nuts and spindle nut lock washers.
My guess is that you may have problems finding Papco brand in Australia, but might be able to track down either Euclid or the parent ArvinMeritor at one of your truck service shops. If you find them as a continuous cone and want a split cone - no problem - just get out a very thin cutoff wheel and make yourself a slit in one side of the cone.