Kosta Barbarouses and Tim Payne scored first-half goals before Oskar Zawada's late penalty pushed Giancarlo Italiano's side to a club-record points haul and highest-ever finish.
But whether that will be first - and a maiden premiership - or second, they won't know until Wednesday.
A Central Coast loss at Newcastle on Saturday would have earned Wellington their first trophy in club history but the Mariners' Â 3-1 win means they can overtake the Kiwi side after their rescheduled match.
Wellington boast 53 points, with the Mariners on 52 before Adelaide United's visit. A point will be enough for Central Coast to be premiers.
Given that fate, for Italiano and the 15,428 Kiwi fans at the Caketin, their victory was bittersweet.
"It leaves a bit of a weird feeling," the first-year coach said.
"I'm not going to watch the game with high expectations. I'm not thinking Adelaide are going to do us a favour."
"It's whether Mariners have the discipline to finish the season with a win and I don't see any sort of evidence that suggests that they're not going to."
In their 17 years, Wellington have not been within touching distance of a trophy, with the Premiers Plate flown over the Tasman and stowed away under the stadium in case their time had come.
Plans for the trophy to be on display or in the chairman's lounge were scotched when coach Giancarlo Italiano declared he didn't want himself or his team to see it, calling it "kryptonite".
Wellington produced a first half worthy of title-winners, bossing the finals-bound Macarthur side.
The Nix pushed and probed around the box, before going route one: a long ball delivering a 21th-minute opener.
Nicholas Pennington produced a superb pass from the deep to play Barbarouses through, the forward taking a touch and burying a record-breaking goal.
Barbarouses' 13th strike of the campaign was his 93rd A-League goal, placing him fourth in the all-time league list, and the top-scoring Kiwi after overtaking Shane Smeltz.
Just before the half-hour mark, Payne doubled the lead with a powerful back-post header from Ben Old's chipped cross.
Like Barbarouses, Old was enjoying far too much space, and might have scored himself, shooting a good chance straight at Filip Kurto and hitting the bar from a cross earlier in the half.
Old's trickery produced the third, dribbling into the box and laying off to Zawada, who went down under a rash challenge and converted an injury-time penalty.
"There was a 35-minute stretch where we completely dominated ... it could have been three or four nil at halftime," Italiano said.
Macarthur - playing for third - enjoyed a decent spell at the top of the second half but were otherwise lamentable as they slipped to a third loss of the campaign to Wellington.
Mile Sterjovski's side will now finish fourth, or fifth if Sydney FC defeat Perth Glory on Sunday, which would rob them of a home final.
"Five changes for our line-up didn't help," Sterjovski said of Saturday's loss.
"The boys are still hungry to finish the season well in the play-offs ... whether we're at home or away, we don't mind."
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