Season chicken with 1/2 tablespoon chilli powder, salt and pepper. Heat oil in a large frying pan or wok over medium high heat and stir-fry seasoned chicken for 3 to 4 minutes, or until no longer pink. Remove from frying pan with a slotted spoon and keep warm.
In same frying pan, stir fry green pepper and onion until soft. Add chillies, tomatoes and remaining 1/2 tablespoon chilli powder. Stir-fry for an additional 3 to 5 minutes; add chicken and stir-fry for 2 minutes more.