A hat-trick to Cowboys forward Jeremiah Nanai and five-star performances from Valentine Holmes and former Bronco Tom Dearden led the visitors, who set up a 14-6 lead in the first half before going on with the game in the second.
It was a sweet homecoming in his second game against the Broncos for Dearden, who scored a try and set up three to outplay Broncos halfback Adam Reynolds.
Reynolds' first match at home was a mixed bag with the former Rabbitohs kicking the ball out twice on the full in the minutes leading up to Holmes' try in the 45th minute.