Going 1-0 down to a Horowhenua field goal in the 10th minute, Wanganui immediately struck back two minutes later as Ryan Gray latched on to another penalty corner to tie the match.
Then after 40 minutes, Remi Halpin connected with a field goal to put them ahead and shut their provincial neighbours out of the contest.
"The result could have been a lot more," said Spence.
"We dominated the game, we held possession of the ball.
"The opposition had to run at us. They struggled to keep up with the way we played."
Wanganui will now meet West Coast in the final after the Coasters accounted for Canterbury 3-0 in the other semifinal.
"They've got some pretty big boys in their team," said Spence.
They then beat Manawatu 3-0 in the quarter-finals.Spence is looking to his experienced midfield players, who still remember the heartbreak of missing out on the final in 2013, to lead the team today.
The final starts at 12pm.