He was undone shortly after breaking the milestone, dismissed for 105 off just 102 balls.
Christopherson was dismissed shortly afterwards for 43 before a unbeaten 16-ball 38 from Kyran Dill propelled Rodney to 231 for 5 from their 40 overs.
Mid Western got off to a tough start, losing three wickets in the opening 12 overs, and they never quite recovered.
Neal Parlane provided a fighting 41 while three players who scored in the twenties but it wasn't quite enough as Rhys Jones (3 for 5) ended their reply on 155 to give Rodney the 76-run victory.
Meanwhile Whangarei cruised to a 98-run win against Duracrete Products City after a fine batting performance.
Whangarei's top order have been searching for substantial runs in the early stages of the 2017/18 season and they found them at Kensington Park with Brad Kneebone, Ben Hyde and Todd Beehre all contributing.
Kneebone set the platform with a well-crafted 57 to open the batting, before Hyde (74 off 59 balls) and Beehre (40) put on a century stand for the fourth wicket.
Hyde's knock included four boundaries and five sixes as he found his groove. Kieran Nelson was the pick of the City bowlers with 2 for 37 as Whangarei managed 253 for 6 from their 40 overs.
In reply, City lost wickets at regular intervals as Whangarei bowled with control.
Sam Sweeney (3 for 17) took early wickets to set them back as all bowlers contributed to the wickets column.
Quinn Pooley was a standout figure for City with a fighting 59, while Simon Fenwick added some late fireworks with a quick-fire 23 not out.
Rodney will now take on Mid Western in the Dargaville Shield final.
