The dismissal of Watson started a consistent fall of wickets but Maungakaramea managed to keep the runs flowing and were 232 for 9 after 49 overs before a curious decision by Onerahi skipper Brad Kneebone.
The skipper threw the ball to Chris Harrison to bowl his first over of the day. Andrews took full advantage in taking 30 off the over including four huge sixes and a boundary.
In response, Onerahi started watchfully as Kneebone (68) let the opening duo of Neal Parlane and Andrews dictate.
Runs eventually started to flow for Onerahi but so did the wickets. Spinners James Merry (3-20) and Scott Harris (2-16) were introduced early which proved to be a match winner as Onerahi were bowled out for 175.
The result of the match makes for an interesting final next week, as both sides deservedly take on each other for the title of one-day champions.
In other premier cricket matches, Boys' High came out on top in their match against Kaipara Flats with the home side asking Boys' High to bowl first.
After their great win last week in the Twenty20 final, Kaipara got off to a good start and were 22 for none after the second over. But the ever reliable Nick Stobart firstly dismissed Finlay Buchanan and then Rory Christopherson in quick succession.
Matt Taylor (33) and Mike O'Flaherty (39) showed some steel with the bat but Kaipara were eventually dismissed for 133 in 37 overs.
In reply, Boys' High lost the early wicket of Dave Armitt off the bowling of Taylor which brought Sam Clotworthy (57) together with Fletcher Coutts (25), the pair adding 83 for the second wicket.
The Kaipara bowling unit, led by three of the Northland bowling attack in Taylor, Forde and Mike O'Flaherty, made the Boys' High batsmen work hard for their runs.
But the slow scoring rate didn't worry Boys' High as they reached the target with five wickets down in the 47th over.
In the final one-day game of the weekend, City hosted Kamo at Cobham Oval in the third premier match of the weekend and the game was tightly contested.
Kamo, batting first, scored a powerful 254 before City fought hard on their way to falling five runs short.