From there, the big forward pack did the rest with two drive-over tries, including the matchwinner in the 75th minute after Whanganui had to play most of the final quarter with 14 men, when lock Peter-Travis Hay-Horton became the newest member of the 2022 "Card Club".
Still unable to play up-tempo at home due to the heavy weather, Whanganui's narrow loss came down to the little moments.
Early on they struggled to contain South Canterbury's ball runners like No 8 Siu Kakala, giving up a try from a charge-down to winger Kalavini Laetigaga not long before halftime.
They didn't get the call for another kickable penalty from some potential foul play when leading 24-22 early in the final quarter, and then ultimately Hay-Horton's red card saw Briggs boot the visitors back in front in the 65th minute, and leave Whanganui short when their forwards drove in the winning try.
There were also a number of handling errors, but these were understandable in the face of hard tackling in the slippery conditions and neither side was immune to it, while South Canterbury produced some great cover defence to twice deny Whanganui hooker Roman Tutauha having a chance to score in the corner off breaks set up by player-of-the-day flanker Ben Whale.
Other standouts for the home side were young prop Keightley Watson in his starting debut, while flankers Samu Kubunavanua and Jamie Hughes worked overtime, and winger Tyler Rogers-Holden made some outstanding cover tackles around the bootlaces.
Centre Timoci Seruwalu scored a breakaway try from halfway, while winger Alekesio Vakarorogo and No 8 Semi Vodosese showed great strength to score their tries through multiple tacklers.
Whanganui were philosophical about the loss, with their bonus point leaving them in second spot - but now having to work hard in their last four games to keep it, and secure a home semifinal at least.
"It is disappointing, no one's hiding from that fact, but there's enough positives to give us more work-ons, than just be too down in the dumps about it," coach Jason Hamlin said.
"We had two or three lineouts down at their end, and we just weren't accurate enough, and that first half cost us.
"Not just in tries, but [that] gave them a lot of confidence. They didn't have to work very hard to get them, and that was the killing part about it all.
"We gave them cheap penalties and cheap possession, and you can't give that to a young guy like Briggs - he kicks those goals with his eyes closed.
"The discipline stuff around losing a man to the bin hurt us and that just gave them a little bit of momentum to get over.
"The things we talked about were being physical, being present, and for the most part we did that - being in the fight.
"I was really impressed after halftime with how they came back. Our second try was just all composure, ball control, all the things we talk about.
"We made a very good side have to play very well to beat us."
Captain Dane Whale knew his side was not that far off - but being a little far off, against a team which has won 20 straight games against Heartland teams and gave Ranfurly Shield holders Hawke's Bay a fright, is still too far.
"Giving them piggybacks down into that five-metre to 10m zone, and they were just able to drive three tries over," Whale said.
"We know where we need to be at that level, but I suppose our discipline just let us down, and the little ones [moments] - if you can put them away, then you're not down in that position.
"Back to the drawing board. There's still a good culture in this team, so one loss isn't going to put us down the toilet.
"We just need to keep working, keep that culture, and work into next week."
South Canterbury 30 (L Simote, C Anderson, K Laetigaga, unknown tries; S Briggs 2 pen, 2 con) bt Whanganui 24 (T Seruwalu, A Vakarorogo, S Vodosese tries; E Malo 3 con, E Robinson pen). HT: 19-14.
