The Heart continued their rise towards the top of the table as first-half goals to youngster Eli Babalj and Mate Dugandzic, followed by second-half strikes from substitutes Jason Hoffman and Alex Terra, inflicted Sydney's worst-ever home defeat.
The win left the Heart just two points behind leaders Central Coast Mariners.
Babalj got the ball rolling on 32 minutes when skipper Fred pounced on some lacklustre defending to steal the ball away and set up his yong team-mate.
Heart were 2-0 up just before half-time when Aziz Behich released Rutger Worm down the left and the Dutchman put in a perfect cross to give Dugandzic an easy tap-in.
The visitors then made the game safe on 76 minutes when Hoffman converted after a counter-attack before home goalkeeper Liam Reddy inexplicably dropped the ball after making a save, to allow Terra to net a late fourth.