What does "Good on ya mate" mean?
It means well done or good for you. It is praise between mates, often for effort as much as for the result.
The saying in a sentence
The saying
Good on ya mate
Example
You passed your exams? Good on ya mate.
When you use it
Use it when someone has done well, tried hard, or coped with a rough patch.
When you do not
It is too casual for a formal reference or a job review. Write well done instead.