Yeah its under intake but above the starter motor hidden away there, it just unscrews and you screw the new one in and plug it in, but being that it is the 2nd time you've had the fault, i'd be checking the wiring to it, and giving the plug a clean while your at it.
You can check the knock sensor with a multimeter it should be under 10k, its basically a microphone in a resonance chamber designed to sound an audio style output when knock is heard, and your fault saying high means that its open circuit i would assume, so you should be able to tell that fairly easy.
I'll just add that on earlier models, not sure about AU, but inside the
ecu is a little bored that soldered off on an angle, it is the knock sensor board, and to me they do look like they could get damaged due to normal driving conditions, it just hangs off to the side on a right angle with soldered pin's holding it in.