Their first half try-scorers were Eddie Tataurangi and Tex Seduadua, with Reihimana Meihana adding a conversion and penalty, while Brooklyn Herewini popped over their last kick.
Celtic's try-scorers were Toby Millar and Kaakaa Kairimu, with the veteran Steelie Koro adding one conversion.
On the other side of the WRFU catchment, there was no contest as Kelso Hunterville swept into their 10th Senior final in 12 seasons, over-running Gemini Pepper Construction Kaierau 42-3 at the Domain.
Hunterville scored two tries through centre Ihimaena Patea and prop Jamie Brett, before Kaierau closed the gap to 10-3 through a Tyrese Lama penalty.
But that was as close as the fourth placed visitors would get as Scott Illston and second-five Blair Osborne, the former Kaierau junior, added unconverted tries for 20-3 at the break.
Brett added two more tries for his hat-trick, while winger Matthew Horton also dotted down twice, and veteran Scott Parkes came off the bench to finally land a successful conversion.
Senior championship semifinals
At Ratana Pa: Ratana 18 (Eddie Tataurangi, Tex Seduadua tries; Reihimana Meihana pen, con, Brooklyn Herewini pen) Marist Celtic 12 (Toby Millar, Kaakaa Kairimu tries; Steelie Koro con). HT: 12-0 Ratana.
At Hunterville Domain: Hunterville 42 (Jamie Brett 3, Matthew Horton 2, Scott Illston, Blair Osborne, Ihimaena Patea tries; Scott Parkes con) Kaierau 3 (Tyrese Lama pen). HT: 20-3.
Around the grounds
METRO: St John's Whanganui Metro ended the MRU Colts playoff hopes of a brave Freyberg Old Boys in a high-scoring 59-31 shootout at home on Saturday. Metro's 11th straight win means they will host Feilding Old Boys-Oroua in the semifinals, despite FOBO losing 27-20 to third-placed Old Boys Marist, who will now take on defending champions the Feilding Yellows in the other playoff.
COLLEGIATE: It was a third-place finish for the Whanganui first XV at the 95th annual Quadrangular Tournament down at Christ's College during the week. After losing 41-14 to defending champions Nelson College on Tuesday, Collegiate went into the minor final with Wellington College on Thursday, winning 24-15. Nelson went to beat hosts Christ's in the major final, 27-25.
SCHOOLS: In the MRU Premier 1 Secondary School Girls grade, Cullinane College lost 32-5 to Palmerston North Girls High on Wednesday. In the most recent Premier 2 Boys round on July 3, City College first XV and WHS first XV drew 27-all to still second first and second on the table. In Premier 3a, Whanganui Collegiate second XV beat Palmerston North Boys' High fourth XV 53-5.
MAORI U18: Whanganui players Konradd Newland, Logan Kingi and Riana Tamati have been selected for the National U18 Maori Whatukura Development camps, being held in Rotorua from July 18-21. Tamati will also be a key player in the Whanganui Under 18 Girls side who will play Taranaki and King Country in Inglewood on Wednesday and Thursday.
