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Ali Abbas soon increased the home side's winning margin as they strolled to three points against a side who are joint-bottom of the table at the Sydney Football Stadium.
Jeremy Brockie's foul in the box gave Del Piero the chance to open the scoring from the penalty spot on 33 minutes, and the Italian legend soon added to his score with a second from a 20-yard free kick.
Wellington were awarded a spot kick of their own in the early stages of the second half, and Hernandez converted from 12 yards to threaten Sydney's lead, but for only 60 seconds.
Having already netted twice, Del Piero capped his man-of-the-match performance by providing Ryall with Sydney's third at the end of a smart passing move, and another goal followed four minutes later.
Abbas made the victory more emphatic by thumping in a 62nd-minute fourth with his left foot. The result sees Sydney climb above Newcastle Jets and into sixth in the table, while Wellington remain ninth.