Facing Wairarapa on the same Queen Elizabeth Park pitch as this weekend, Wanganui had the home side under pressure at 29-2 last Saturday, but then dropped catches helped them to reach 343, with play rained out the following day.
Marist's opening batsman Chris Stewart dislocated his shoulder in that game.
The visitors made the semifinal by virtue of a first-innings points win over Property Brokers United in the Whanganui derby on November 6 and 20, while United CC got through courtesy of a two-wicket thriller against Burger King Red Star in a one-innings match.
The other scheduled semifinal is a Horowhenua-Kāpiti derby, as Kāpiti Old Boys take on Levin Old Boys in a rematch of the Coastal Challenge Twenty20 final, won by Levin at Donnelly Park.
The P2 40 resumes with games at Tasman Tanning (Victoria) Park and Whanganui Collegiate.
Unbeaten Treadwell Gordon United 2nd XI, who lead the competition with three wins, take on the Kaitoke Knight Riders.
A crucial game will be Wanganui Renegades hosting Matt Burke Engineering Marton Saracens in the match between third and fourth on the points table.
Games start at 12.30pm.
Draw for December 18, P2 40
Wanganui Renegades CC v Matt Burke Engineering Marton Saracens
Tech Old Boys v Wicket Warriors Whanganui
Kaitoke Knight Riders v Treadwell Gordon United 2nd XI
Marist Bs v United 3rds
Whanganui Collegiate Senior Black v Whanganui Collegiate Senior Blue