I dare say the ATSB will start with fuel - did he take off with the selectors on partially filled aux tanks and when the first stopped was he so preoccupied with getting back to YSBK he never checked/changed tanks and subsequently ran the second dry a few minutes later?
I think it's already been stated that the Mojave doesn't have aux tanks. Even if it did, if he was heading for Brissy with only two on board surely they would have all been full?
DF.