Sounds like the typical Sydney approach scenario:
Pilot: "Plastic 320 leaving FL 350 on descent."
ATC: "Plastic 320 make speed 2 5 0 knots."
Pilot: "Thanks for that."
Into Maroochydore, I'd say the pilots were so excited at being able to do whatever they want instead of being nagged the entire approach, they wanted to get low on profile so they could enjoy the view.
But with a dinky little gay speedbrake lever such as an deathstar has, it's just as likely they didn't notice it was in the extended position.
Yet again, it may have been (as someone pointed out) that they were trying to rid themselves of the stench of 150 ripe, unwashed pommy back-packers. Takes a while for the pong to float up the back though, even with that body angle.