Sqad,
They've already amassed an insurmountable total, they just wanted to freshen up their bowlers by giving them an extra half day rest so that they can come steaming in tomorrow, shortly before lunch IMHO.
Not that Mitchell needs any extra rest, he looks raring to go every time they chuck him the ball!
Because the next test in Perth follows quickly after this one and they are resting the bowlers rather than enforcing the follow on. I imagine they won't bat for too long tomorrow and then ......well we'll wait and see !!