The Mariners, who drew twice with unbeaten premiers City in the regular season, took an early lead through Brooke Nunn in the 20th minute of their home leg at Gosford's Industree Group Stadium on Saturday.
But the visitors equalised through Leticia McKenna's stunning free kick in the 29th, before Bryleeh Henry put them in front six minutes later.
Nunn brilliantly drew the Mariners level from a tight angle in the 54th minute.
"We were at home, we were like 'we better go for it'," Nunn told Network Ten.
"And they're a brilliant team, but they haven't beat us this season. So we had nothing to lose, so we put our head down and we really went for it today.
"We're gonna have a big boost from today's result and we're gonna just keep our head down and look for a result."
After backing up a physical elimination final win over Canberra United, Nunn said: "We're holding on by a thread.
"But it's finals football, the adrenaline just gets you going and you want to win, you want to come out with something. So that keeps us going."
The winner of the tie on aggregate will face Melbourne Victory or Adelaide United, who play their first leg at Coopers Stadium on Sunday, in the grand final.
City's golden boot leader Holly McNamara started after suffering a "niggle" in training, while winger Lourdes Bosch (back) didn't play.
"Holly felt on Tuesday a little bit of a knock, so got the scan all clear, so then we didn't quite want to risk her for a full start," coach Michael Matricciani told Network Ten.
"But she played that 25, 30 minutes off the bench, which is pleasing. She got through that well. So she'll be ready next week.
"All in all, on reflection, I think a draw is probably a fair result and we take that."
Mariana Speckmaier (ankle) is due to return while City hope Bosch can play.
The Mariners were without important midfielder/defender Taylor Ray through injury.
Both sides traded early chances before the hosts, led by star midfielder Isabel Gomez, snatched the lead.
Annalise Rasmussen nutmegged Rebekah Stott, drove down the left and slipped through Gomez, who squared it for Jade Pennock.
The Englishwoman's strike was well saved by Malena Mieres but Nunn was on hand to tap home the rebound.
It was the ninth time City had fallen behind this season - all nine times they have come back and avoided defeat.
They equalised when from distance, McKenna stepped up to take a free kick and hit the top corner with a wonderful strike.
Then, Henry went on a wonderful run down the left before rounding Langman to score her fifth goal in eight games.
Nine minutes into the second half, the Mariners levelled.
Rasmussen released Pennock who slipped through Nunn with a wonderful, incisive pass.
Nunn rounded Mieres then cut onto her right and finished from a tight angle.
City unleashed McNamara in the 66th minute but neither team could find a late breakthrough.
