Taranaki replied when Whanganui did not mark-up out wide and second-five Brent Landers swept into the gap, firing a long ball to winger Taylor Patene for 12-5.
Whanganui made good use of their bench depth at halftime, and dominated the third quarter with three tries in 11 minutes.
Centre Troy Brown powered through a gap and assessed his options, putting down a grubber kick for winger Isaiah Hooper to cross over in the corner.
The home side were straight back on attack as Forsythe, now running the cutter, crossed in the far corner, and then halfback Kahl Elers-Green, who had been threatening all match, broke through to dash away for 29-5.
The Maori then scored their best try as Forsythe pounced after a Taranaki fumble to break through at midfield and feed Hooper, who was just dragged down inside the 22m but popped his pass up and a long ball was sent to reserve forward Dale Pene to run to the open posts.
Elers-Green kept waiting for another opening and when Taranaki left him alone at the ruck, he was through and running to the sticks for 43-5.
Taranaki reserve forward Shamus Hurley-Langton scored a consolation try from a drive to the line and Pene was sinbinned for a dangerous tackle from the kickoff, but for the Whanganui Maori, the celebrations had already begun.
It fell to Taranaki captain and former Whanganui stalwart Mark Davis to have the bittersweet moment of leaving the Tuera Shield in his "second home", as he presented the prize to Roman Tutauha.
"You look after this for this year, and we'll give it another go next year."
Proud Whanganui coach Vaan Rauhina wanted to thank Lock, McKenzie and chief executive Bridget Belsham for getting in behind the concept.
In Maori rugby, the ideal is to play a free-flowing style and key to that was the team being filled with experienced representative players who could hang onto the ball when it was being thrown about.
"A lot of it goes to the union and the Development team, because they had a really good season," Rauhina said.
"The union's really strong and fortunately for us in Maori rugby, we were able to reap benefits.
"To be able to select a top team, it just shows the depth in Whanganui rugby."
Rauhina now hoped to keep as many of the current squad together as possible to play summer games.
Whanganui Maori 43 (Kahl Elers-Green 2, Sean van der Lubbe, Kieran Hussey, Isaiah Hooper, James Forsythe, Dale Pene tries; Forsythe 3 con, Te Tua Kemp con) bt Taranaki Maori 12 (Taylor Patene, Shamus Hurley-Langton tries; Brent Landers con). HT: 12-5.