They'll bat on I would think a quick dip after lunch to unsettle the South African openers who can't prepare mentally over a 40 minute lunch break.
If they can get to 450 or when the next wicket falls (even if first ball) then pull out - no point in exposing Broad to any variable bounce.
Australia have made 330 for 6 (so could possibly have made over 400 if needed) to win a game here and plenty of time left to bowl them out.
They have 153 overs left in the game (minus two for change of innings = 151) so bat for half an hour ideally and leave 145 overs to get them out with 450 ahead, although I can't see runs coming into it, its just time out of the game, a second new ball after 80 overs and unsettling their tired team - make them ragged and mess with them mentally by stamping them into the ground.
Hopefully that crack on the wicket and the uneven bounce will be even worse tomorrow than today and Broads height a big advantage.