The Italian limped out of the game after 43 minutes after picking up a hamstring problem, but Joel Chianese scored for Sydney just two minutes with Ranko Despotovic rounding the scoring off on the hour.
Chianese's perfect first touch from Ali Abbas' cross from the left took him past defender Jacob Pepper he fired past Mark Birighitti with a shot on the turn.
It was Chianese's fourth goal in four appearances against Newcastle and Sydney's 300th A-League goal.
Despotovic came on for Del Piero and soon had his second Sydney goal in as many weeks, drifting past a couple of defenders before beating Birighitti at the near post with a powerful left-foot drive.
Elsewhere, Brisbane Roar overcame Perth Glory 1-0 to record a sixth straight A-League win at Suncorp Stadium.
The Perth defence took some breaking down, but Brisbane's attacking pressure eventually paid off, with Besart Berisha's sharply taken goal in the 59th minute ensuring the Roar extend their lead at the top of the table.
Perth created a handful of opportunities to score but were largely outplayed over the course of the match, with the home side enjoying 65 percent possession and firing in 22 shots on goal to Glory's five.
