Necevski made six starts for the Mariners this season after joining on a one-year deal from Sydney FC.
But the 37-year-old was unable to hold the spot ahead of first-choice Paul Izzo during a very an up-and-down season in Gosford
His departure, which comes a day after Izzo's move to Adelaide United was confirmed, appears to have hit the ex-New Zealand Knights and Newcastle player hard.
"Football is a funny game, gives you the highest of highs and the lowest of lows," he wrote on Instagram.
"Being told you are no longer required in any job is never a good feeling especially in the football world.
"Every time i put the that jersey on, be it at training or in games, i gave my blood sweat and tears for the team."
The Mariners now have two goalkeeper spots to fill in the off-season, with reports linking the club to Jets custodian Ben Kennedy.
Okon has flagged mass changes after his first season in charge of the competition strugglers as he takes the opportunity to shape his own squad.
The club were disappointed to lose striker Roy O'Donovan to the Jets and will potentially farewell Portuguese attacker Fabio Ferreira should links to Western Sydney prove true.
Captain Nick Montgomery is also out of contract and no guarantee to stay while Okon appears unlikely to re-sign Senegalese cousins Jacques Faty and Mickael Tavares.