I've asked this question going back since the start of this GCA.com DG. Some of the answers that were given back a decade ago was both the soil particle size and nature, and the root system of the "couch" turf, which is I guess a cultivar of bermuda turfgrass. It was noted by some of the Aussie contributors that the exacto knife sharp edges that we see on TV when tours are broadcasting events in their season from Oz is not the same level of crisp sharp bunker edges year around. (I have no idea to what extent that is correct).
One thing in Ryan's example photos that seems to me observable from those pics (accurate or not?) is that all their bunkers with the crisp edges aren't as dramatically cut straight down into the root zone, and the very edges of those newer bunkers seem like the last foot or so are banked or built up, whereas the one in Oz seem to fall right in with no last bit of edge sloped up away from the fall-in.
But, it makes sense to me that having a very strong, almost jute-back fibrous root system of turf, in sand-loam that is not so prone to crumble when exactly cut, is a fair theory to assume. Also, that would then go obviously to what sort of irrigation they apply, and what amount of rain they get that tend to break down or wash out that exacto cut through the soil-root zone.
At any rate, I am among the folks that love to see 'the look' of it when the cameras turn to Oz courses. It seems that Barney Dunes and Lost Farms (from photos) have decided to follow the Sand Hills of NE prairie approach to maintaining the lips rather than the Melbourne or Oz traditional regimine; would that be correct Tom?
I'd think Mr Clayton would have the most insight into all of this if he'd care to explain.