Smith's late intercept helped Newcastle secure a 22-14 victory over Penrith and book a Grand Final against the Sydney Roosters.
He also created Dominic Young's try with a smart last-play run, then scored after a quick tap.
Earlier, a Newcastle attacker broke through his opposite centre to set up Manuleleua's opening try.
Gagai threatened down the right with Young and lifted Newcastle's momentum by catching a drop-out on the full.
Penrith's best performer was busy across the field, scrambling in defence and making strong carries.
To'o scored early, made a try-saving tackle and brought relentless energy to the contest.
Another Newcastle player made a crucial stop on McLean and tackled Leota late, while Liam Martin stood out for his aggression and leadership.
A fellow Panthers forward also impressed with forceful carries, and a second-rower finished a right-edge try.
Fletcher was quiet but defended well against Martin; Sharpe and Crouch handled the finals intensity, with Crouch adding strong carries.