The home side made a strong start to the match as Alessandro del Piero put them ahead after 11 minutes with a powerful strike from outside the box.
Petratos stepped up to take the spot kick, coolly tapping the ball in for his first goal of the day, which gave the Roar the lead for the first time.
The winger was able to extend the away side's lead further before half time as Broich pulled the ball back to Petratos, who drilled it in from the centre of the box.
Sydney fought hard at the start of the second half, forcing a series of saves from Michael Theoklitos, but the Brisbane keeper was able to stop the hosts getting back into the match.
Petratos was handed the chance to complete his hat-trick with just over 10 minutes left to play when Nikola Petkovic pushed Broich down in the area, giving the Roar their second penalty of the game.
The 21-year-old went to the same side as in the first half, but this time powered the ball into the roof of the net to beat Vedran Janjetovic from the spot again.
Ivan Franjic made it five in stoppage time, picking out the top corner with a curling effort from the edge of the box.