Machinery's Handbook, (the engineers bible) recommends poplar as the best for wooden brake blocks. To quote, "Poplar gives a high coefficient of friction, and is little affected by oil". It then goes on to list in order of effectiveness, poplar, oak, beech and elm but warns elm drops off if affected by oil. It is also careful to say "In all case the motion is in the direction of the fibres in the wood."
Of course it's a US/English publication and all of these are common Northern Hemisphere woods that their engineers would have no problem sourcing with no mention of Australian hardwoods.
Can I ask why no one seems interested in the blocks of 'Retarding Medium' the English VMCC sells? Apparently they had legal issues if they called it brake block material.
Cheers, Bob