Wanganui's pairs team Ian Porteous and Bernard Anderson won three games, while singles competitor Bruce Winterburn won two.
That earned them 22 overall points ahead of Dunedin with 15 points, plus a superior differential to Bay of Plenty and Southland.
Among the remaining eight teams in post section, Wanganui were drawn to face Canterbury, the home team and 2014 champions.
"It was close fought with our boys coming through, winning the pairs and fours putting us through to the semifinal. Great news and we are elated," said Morgan.
Going against Taranaki in the semifinals, only Porteous and Anderson could deliver a win in the pairs as Winterburn lost while Park's fours team were beaten for the first time.
Taranaki went on to beat Marlborough in the final with victory in the singles and pairs games.
"A great effort by our team who are to be congratulated for their efforts," Morgan said.