I would repot the Japanese Maple and remove the excess soil on the top of the root ball until you see the roots. The top of the root ball should be even with the outside soil.

If located anywhere in the northern hemisphere, leaf drop is to be expected at this time of year. Since I seem to recall you are "down south", I'd follow the above recommendations. Also, make sure your potting medium is very well draining and heavily bark-based rather than peat-based. It may be retaining too much water.

Looks like you found the main problem.

Yes im I australia, nearing the end of spring heading into summer soon.
I meant to write drooping, as in saggy :)
Anyway I have repotted, loosened it up and added a bit of perlite to improve drainage. Hopefully I can save it.

Looks like it was doing great prior to the mechanical damage, as though that is the only critical problem. Everything depends now on the remaining strip of bark being enough to keep it going. Or it being able to sprout new shoots below the zone of removed bark - the current wilting suggesting that ability to maintain a top above the wound has been lost. If it seems there is an animal present that gnawed it you might want to install some kind of trunk protection.