The home team began strongly, asserting dominance in possession with a raft of Junior Football Ferns bound for the U-20 World Cup in Canada flexing their muscle.
Despite threatening, Forrest Hill couldn't turn that possession into goals until Daisy Cleverley tapped in a cross from CJ Bott four minutes into the second half.
However, Coastal Spirit stood up to the challenge and, using their press effectively, began to establish a foothold for which they were rewarded on the 60-minute mark.
Seconds after whipping a delightful freekick against the crossbar, fullback Laura Merrin delivered a corner which former clubmate Lily Alfeld could only turn into her own net.
Despite wobbling slightly, Alfeld's Forrest Hill continued to hold the edge in possession and a number of chances came and went.
With extra time only seconds away, Megan Lee robbed Merrin and, after an initial shot from Lee was saved, CJ Bott put the ball on a plate for Rolston who applied the telling touch to a raucous cheer from the home supporters.
Forrest Hill Milford United AFC coach Mauro Donoso was a happy man at the final whistle, his side getting across the line with a victory he felt they deserved.
"It's good for the girls and we've been working very hard for this moment," Donoso said.
"We dominated the ball and created so many opportunities and we're all very happy that we scored the goal late."
The focus now returns to league action for both sides in the Northern Regional Premier League until the ASB Women's Knockout Cup Final on September 7.