City began the match the stronger side, with Fornaroli and Connor Chapman going close in the opening 15 minutes. City continued to press forward and were rewarded in the 26th minute when they opened the scoring.
After goalkeeper Thomas Sorensen kept out Roy O'Donovan's effort, City hit the Mariners on the counter-attack, which saw Fornaroli put Novillo clean through on goal, and the French striker slid the ball beyond the advancing Paul Izzo to give his side the lead and score for a third game in succession.
They doubled their lead just before the break, as Fornaroli played a clever dummy and one-two with Erik Paartalu in the penalty area before slotting home off the post.
City continued their dominance after the break and, on 56 minutes, Novillo netted his second and City's third of the contest as he beat Izzo with an accurate drive from 20 yards.
O'Donovan almost pulled one back for the Mariners, but Patrick Kisnorbo was on hand to deny the Irishman with a sliding challenge as he broke through on goal. Ivan Franjic was then on hand to deny the Mariners again after O'Donovan had slid the ball across the face of goal, with Mitch Austin firing over.
Melbourne put the result beyond doubt 15 minutes from the end as Stefan Mauk fired the ball beyond Izzo after being picked out inside the area by Aaron Mooy.
The Mariners netted a consolation four minutes from time through Austin, who converted a penalty after he was brought down in the box, but City added a fifth through Mooy in injury time to secure a comfortable victory and their first at the Coast.