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First-half goals from Anthony Caceres, Daniel McBreen and Matt Simon wrapped up a convincing triumph for the defending champions on home soil.
The result means Newcastle have failed to win a league fixture in Gosford since January 12, 2008, losing six matches and drawing three in that period.
Caceres set the tone after 13 minutes, unleashing an unstoppable 25-yard drive that flew past goalkeeper Mark Birighitti.
The Mariners doubled their lead after 32 minutes when McBreen seized possession on the edge of the area and arrowed a left-footed shot into the back of the net.
And the victory was complete when Matt Simon headed home Mile Sterjovski's corner in first-half stoppage time.
Ryan Griffiths' stunning last-gasp goal denied Melbourne Heart a second consecutive win as Adelaide United stole a 2-2 draw.
After Bruce Djite's 22nd-minute strike gave the Reds the early advantage, Harry Kewell coolly capitalised from the penalty spot in the 61st minute following a Nigel Boogaard's foul.
Former Leicester City and Leeds United stalwart Patrick Kisnorbo scored a superb goal 18 minutes later to give Heart the edge but Griffiths' excellent volley from Marcelo Carrusca's corner ensured the hosts had the last word.
