Diesel used to be the less refined product. More energy, yes, so generally on a normalized basis, it is a superior deal... but it doesnt change the fact that it is a less processed product, and had less of a cost associated with it.
Now in the days of ULSD, what happens is that the diesel buyer pays for the capital cost of all those $$$ HDS units required to perform deep desulfurization of the fuel. it is not cheap.
Plus, crudes are benchmarked in a way associated with the processing (i.e. hydrogen, $$$) required to perform such cleanup. Good sweet crudes command a premium because they dont need as much work to meet the ULSD spec, the heavier, nastier ones, need a lot more HDS to clean up... so not only do you have to build the units to accept high-sulfur product, but also spend the hydrogen to perform the HDS reaction.
Nothing is cheap. If we still had 500ppm diesel, it likely would still be on par with gas (other than during heating season, when it usually bumps up). There are other forces at work, first and foremost companies charging to pay for the upgrades they need to make product.
Not sure what grade diesel Australia uses, or if it is the same factors at work. Taxes take such a chunk from the finished, fully burdened price, that sometimes they can make things very unclear.
JMH