A policeman accused of assaulting a teenager has told a jury that he punched the boy in the face because he was struck first.

Senior Constable Simon Mareangareu took to the County Court witness box on Wednesday and told his trial he punched Kyan Foster about 2am on Christmas Day 2014 because the then 16-year-old hit his arm.

Senior Constable Simon Mareangareu. Credit:Eddie Jim

"As I have got his backpack strap ... he has whacked me in the arm,'" Senior Constable Mareangareu said.

"He hits, I react. The training instantly kicks in. Create distance. Push, punch, kick, do anything. Create distance.