This appears to have happened after Jackson was thrown to the ground the first time (after which a bystander can be heard accusing the police of "whacking" Jackson's head against the concrete hard enough to leave blood behind) but before Jackson was thrown to the ground, cuffed, hitting his head against the concrete a second time.
Video of the second fall has been seen over a million times on YouTube. Australia's 9 News obtained and aired the new footage.
Here is part of 9 News' report, which makes no mention of exactly how Jackson ended up on the ground the first time:
Out of the shadows of Oxford Street, new video, showing the moments before handcuffed Jamie Jackson was thrown to the ground. As the two officers attempt to cuff him, Jamie throws a punch. One officer then punches Jamie in full view of a shocked crowd. On the ground, weighing in a 60 kilos, Jamie fights back and attempts to kick the two police officers.