The Raiders played champagne rugby league in Dubbo and scored four first-half tries in racing to a 20-0 lead before hanging on to win 32-12 as the Rabbitohs lifted late.
The win really breathes life into the Raiders' season, now 5-6 after their campaign had earlier looked on life support, while the Rabbitohs boast that same record after losing three of their last five games.
Fittingly, gun prop Papali'i made an early impact in his milestone match by powering over for a trademark try, shortly after returning star Jack Wighton had created an opener for winger Nick Cotric.
The Raiders' forwards looked hungry early and delivered plenty of big hits as they took control through the middle, with a huge goal-line shot from Papali'i denying Liam Knight points at 8-0.
They again struck through the middle when hooker Tom Starling put a try on a platter for Hudson Young, before an electrifying team move saw a Corey Horsburgh flick pass create a line break that let Seb Kris cross to cap an outstanding half at 20-0.
Any chance of a patented Raiders fade out seemed put to bed early in the second half when a Jordan Rapana flick put Matt Timoko through, although the Rabbitohs finally got on the board on 52 minutes when hooker Damian Cook broke three tackles and scored for 24-6.
But Canberra withstood the surge and sealed it through star youngster Xavier Savage, who flashed his pace and sliced through the Souths' line for his first try of the season.
Papali'i ran for 116m to go with eight tackle busts and his try, while another 174m and 86m post-contact from Joe Tapine also helped them control the middle.
It was a lamentable display from Souths, completing just 61 per cent of their first-half sets and allowing a staggering 30 tackle breaks in that period too.
They'll end the round eighth on the table but teams are queuing up to remove them from the finals spots, with Manly, St George Illawarra and Canberra all also on five wins.
Tevita Tatola was taken from the field after the first play of the match having taking a head knock from a driving Papali'i run, and Taane Milne also failed a first-half HIA.
Canberra fullback Charnze Nicoll-Klokstad left the contest at halftime with a hamstring injury.