Well whowever wired that up needs a very large kick in the appropriate place The black wire is a mess for a start The exposed wire on the red and brown looks to long as well. The usual reason this happens is because some one hasnt tightened the screws enough or the user pulls it off using the cable instead of the plug, or both.
Look here where you can see what is what in different plugs 6 pin trailer plug wiring diagram australia - Google Search
Or here if its a 7 pin
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