A controversial Marco Urena penalty proved the difference as the Central Coast Mariners sealed their first FFA Cup final spot with a 1-0 win over Sydney FC at Kogarah Oval.

After almost a month off due to COVID-19 postponements, the Mariners sealed victory in the 82nd minute when Urena fired past Andrew Redmayne for the winner.

But every bit of evidence suggested referee Alex King should never have pointed to the spot.

Lewis Miller had made a charging run down the Central Coast right and as he approached the box there was a light clip from leftback Joel King.

Replays showed the foul happened outside the box, but with no VAR on hand - as there is in the A-League Men - Sydney were at the mercy of the referee's decision.

He awarded a penalty without hesitation and Redmayne, who had kept Sydney in the game with three close-range denials of Brazilian striker Moresche, was powerless to stop Urena's strike.

"I don't think it was a penalty, I thought it was outside the box to be honest," Sydney striker Elvis Kamsoba said.

"Both teams had enough chances to score, but if you don't score you don't win games and you get punished for that."

Kamsoba had Sydney's best opportunity of the first half but Mariners centre back Dan Hall was on hand to block the close-range effort.

At the other end, Redmayne's heroics denied Moresche, Beni Nkololo and Urena from giving the Mariners the lead.

Milos Ninkovic and Trent Buhagiar had chances for Sydney, but King's challenge on Miller gave Urena the opportunity to convert from 18 yards.

Despite Redmayne skipping his feet and waving his arms, the Sydney No.1 was unable to stop Urena as he blasted the ball down the middle to send the Mariners into the final.

"I'm really proud of this team," Urena said. "We wanted to make history and we did it. We deserved to win no matter how."

The Central Coast will now play Wellington or Melbourne Victory in the final for which a date has yet to be determined.