Commonwealth, state and local elections are held separately.
How many candidates for hotly contested races?
For Senate elections it is not uncommon for there to be literally hundreds of names on the ballot, which is why "above the line voting" is an option.
How many ballots are procssed?
For a Commonwealth and State elections, millions, with an indicative count (ie, whether there a clear winner on first preferences) made on the same night by tallying across hundreds or thousands of polling stations.
More to the point: it scales. The indicative count is easily parallelised to any level, since summation is a commutative operation.
I think your implication is that Australia doesn't do it at a large enough scale and that therefore, our experiences don't count. That isn't true. Commonwealth elections are comparable in scale and complexity to the operations of those run by the largest states in the USA to handle federal elections.
If you're handling large scale, complicated IRV elections with ease, than that should inform the efforts of our local proponents (eg fairvote.org).
My jurisdiction often has 30+ contests. We had 22 belligerents for mayor 2 years ago. I think we had 12 for senator in our most recent primary. I dimly recall my county counts 450k ballots.