Ulverstone will play its first final at home in five years after moving above Smithton on the ladder yesterday.
The Robins produced a 13-goal second half to eclipse Penguin and earn the right to a home final.
Smithton's chances of retaining third place on the ladder took a dive when Latrobe ended the roster season unbeaten with a 60-point win.
NTFL president Andrew Richardson last night confirmed Ulverstone and Smithton would meet in the first semi-final at Ulverstone next Sunday, while Latrobe and Wynyard would meet in the second semi-final at Latrobe on Saturday.
The Robins appeared to be struggling against Penguin and went into the long break trailing by 15 points.
In the third term, the Ulverstone fightback began, mainly through the efforts of small forward Jordan Kelly.
Kelly kicked five goals after half-time to erase the deficit and then allow Ulverstone to take the lead and steam ahead.
Ulverstone had good players appear from everywhere, with captain Jarryd Chatwin, midfielder Jeremy Soden and key forward Josh Walmsley taking over.
Walmsley, who returned to the team after resting a knee injury for a couple of weeks, put in his nomination for mark of the year when he flew above the pack at centre half-forward late in the third quarter.
Walmsley came down safely with the ball and then went back to slot one of his three goals for the game.
Penguin appeared on song for an upset, with present coach Justin Cotton and previous leader Jason Ling causing damage in the first half.
Cotton kicked three goals in the first half and seemed headed for a big tally before Ulverstone defender Ryan Wing was placed on him.
Penguin's chances were limited in the second half when Jacob Brown (suspected broken jaw) and Rodney King (hamstring) couldn't take the field.