If Martin has a reasonable belief that Zimmerman is threatening him, and attempting to draw a firearm while going so, then it is indeed lawful for Martin to bash Zimmerman's head against the concrete sidewalk.
It does appear from a map that has been posted on FR, that Martin could easily have gotten to the safety of his father's home after he noticed Zimmerman. He told his girlfriend that he wouldn't run--would he say that if he thought his life was threatened? Anyway he has no reason to suspect that Zimmerman has a gun at this point.
If, from what we have heard of the case so far, Martin had a lawful basis for bashing Zimmerman's head on the sidewalk, then Zimmerman has totally misrepresented what happened...or the rules for lawful self-defense are much different in Australia than they are in the US.