Hi Albie
It looks like an injury to me, that caused aninfection. I do not think it is a bacterial infection that caused the injury.
However, I suggest you observe the other fish carefully to see whether you are having a bacterial problem in the pond.
Sedate the fish a bit with clove oil. Put in on a wet towl, clean the wound with Hydrogen peroxide. Dry it with toilet paper. Add a bit of Malachite green mixed with terramycin powder to the wound. Cover with Friar's balsam to "waterproof" the wound. Blow it dry with a hair dryer set on cold. Put the fish back in the pond where there are a lot of oxygen to allow the fish to recover from the clove oil.
Repeat every few days until you see an improvement.
The Hydrogen peroxide is not crucial if you do not have.
Increase salt levels to 3kg/1000 liters.
If you have quarantine facilities/hospital tank, use that.