TIMELINE OF AUSTRALIA'S DAY-FOUR DISASTER
* 09:45: Travis Head square-drives the first ball of the day from Kagiso Rabada for four and it all looks rosy for Australia.
* 09:51: After Nic Maddinson's duck brought him in as a nightwatchman on night three, Scott Boland wafts at his first ball on Wednesday is caught behind for two.
* 10:12: Marco Jansen gets Head on the pads with a ball that seams back a long way. It's given not out but overturned on review, and the cracks are showing.
* 10:33: Simon Harmer takes a one-handed screamer at slips to remove Marnus Labuschagne for 14. Australia are shaky at 3-22.
* 11:33: Steve Smith edges a ball off Rabada and flings his head around to see it bounce in front of slips. But in doing so he hurts his neck and requires treatment.
* 11:40: Smith looks a completely different batter after his neck issue. The right-hander struggles through Rabada's next over.
* 11:47: Matt Renshaw's 48-ball resistance alongside Smith ends when he edges Jansen to Harmer at slip on 12. Alex Carey follows two balls later for a duck and it's 6-65.
* 13:04: Any hope of a revival after lunch is over when Beau Webster edges Jansen to slip.
* 13:22: Smith receives treatment on his neck again, taking pills for a second time. Then four balls later he is bowled by Wiaan Mulder for 28.
* 13:27: Mitchell Starc is caught behind off Rabada and five minutes later Pat Cummins is the last man dismissed for seven. Australia are all out for 99 and are asked to follow on for just the third time since 1988.
* 13:54: Following on, Maddinson is caught behind off Rabada for eight to start the second innings, and his shock Test recall is surely over.
* 16:03: Labuschagne is given out lbw after fighting his way to 36, but reviews. Ball-tracking shows the Harmer delivery missing leg stump by 4.7 millimetres and Labuschagne reacts like a batter who knows his career was in the balance.
* 16:18: After taking Australia to 1-100 alongside Head, Labuschagne attempts to hit Harmer into the wind and is caught halfway to the long-on boundary. He admonishes himself as he walks off, his future now in the hands of the selectors.
* 16:49: Renshaw is bowled trying to leave Rabada on 11, and any momentum Australia had clawed back is gone again.
* 17:25: Webster is lbw to Harmer on eight. His spot is now vulnerable, after averaging 13.5 over the past four Tests.
* 17:47: After starting the day as nightwatchman, Boland ends it as one again. He helps see Head to the close with the opener unbeaten on 87 and Australia 4-156, still trailing by 170 runs.