Hence my question about whether this was one of those routes where (statistically) 80% of the checked baggage qualifies for priority handling.

That is what QF found on some routes, although I don't know which ones. As you mention SYD-MEL, that seems like a possible candidate.

Priority Baggage
 

At MEL I don't think it matters which airline one uses, priority baggage never comes out in the first batch for me. At HKG, BKK, SGP & PVG my priority tagged bags always come out within the first dozen bags & I've done all of these airports including MEL one or more times every month since 2004. My conclusion is that there's a conspiracy by the baggage handlers at MEL to deliberately delay priority baggage as spite directed at the high status flyers ie good old Aussie toppling of tall poppies.

SYD-MEL / SYD-BNE would have a high priority pax rate, but not at 80% I would have thought.
Fact is that Qantas are crap at delivering on the benefit. I love travelling to China for example. Despite me OW status with a 95% certainty my bag will be inside the top 10

CBR! Lot's of statused PAX flying (and replenishing status but generally not earning Frequent Flyer points).

Well i flew QF93 today MEL-LAX and yet again my bags were not tagged at all. The BP says FF EMERALD AA####### but obviously their system must be so glitchy or its not programmed to deliver this oneworld benefit just yet.

Either way i was highly dissatisfied with my QF experience after being a longtime UA flyer to aus. I honestly should not have to wait 40 mins for my bags

problem with some australian ports is even if your bag is tagged priority, it doesn't seem to be treated as priority!

I fly over the ditch to MEL few times a year, OOL at least once a year and occasionally to SYD, and never had issues with priority baggage handling with NZ/VA. Always have Star Alliance or VA tags on properly at check-in and bags almost always there waiting for me, or less than 5-10 minutes wait at most once I clear immigration.

In fact I'd say my bags being >20 mins wait is the exception to the norm (once every 6-7 flights at most?)

Perhaps its something QF or OW in general just lacks discipline?

IME flying to/from MEL was always the issue with priority.
SYD T1 and non-MEL flights T3 my bags seemed to come out near the front.
MEL I have experienced long waits both international and domestic, in fact I really am not that big a fan of MEL (the airport, the city is a wonderful place to visit:)).
However I must admit I have generally stopped caring or counting as that could of course increase my agrivation levels about travelling.;) I just let it go as life is too short to let these minor things get to you....unless of course bags not coming out after 45 minutes, needing to transfer to a different terminal to check in for an international flight, that only departs 3 times a week....I am talking abut you IB and MAD:mad:

Out of curiosity, has there been any improvement regarding bag tagging over the past year?

I've had two personal experiences over the past few months.

1) Business. Checked in at NRT with JAL, arrived via Manila in SYD. Bag was priority tagged and was #16 on the belt.

2) First, but first flight in domestic J. Checked in at SYD, via MEL and LAX to JFK (AA on the last leg). Bags not tagged (had the useless Business class symbol printed on the tag itself, but that's it). Bags were some of the last ones onto the belt as could be expected.
Didn't help that the guy at check in was in a bit of a mood. Had a better experience with the security screener doing the 'extra' screening than with him.

If this matters to you then fly CX, SQ, EK ... they all seem to get F bags out very fast. QF and AA have always been hit or miss and it hasn't changed much despite it often being raised as an issue. On the plus side most airports have speeded up delivery time for all bags, so being last out isn't as bad as used to be. Priority baggage delivery just seems to be a low priority benefit for many airlines.

If the priority tag means it travels on the same plane as me, then I'm happy that its been prioritised over the bags that might travel on another flight. :D

I don't understand this obsession with priority tags. It's more important to have priority entered onto the baggage tag. Once it's loaded on to the belt and scanned, it will send the bag where it needs to be sent accordingly.

You can have all the oneworld priority tags plastered on your luggage and it means NOTHING to the barcode reader.

You forget that there is a human element in the baggage system. There are some humans who either don't give a rats or they work in systems where there is no want or capability to prioritise baggage and this is not just a problem in Oz airports.