It was a feisty, tense encounter with Wellington's Alex Fenerides sent off in injury time amid a rash of yellow cards for both sides.
As his players indulged in mass hugs and the now obligatory water splashing, a breathless Bay coach Brett Angell said: "All credit to the boys. They've been tremendous all season and they've carried it through again, and rightly and deservedly won the game.
"We're quite disappointed that the goals we conceded were a little bit scrappy but goals are goals and likewise, when we were at the other end, we are always a goal threat.
"And today, yes, we got a couple of penalties. I'm sure Matt will disagree that they were but at the end of day the referee's given them and it's the cool head of Ryan who scored both of them."
Angell felt few would have given them a chance to go through but he had urged his troops to believe even though they had a propensity this summer to concede goals early and yet win."
"They're a confident group and they give everything they've got. They work well together and I always feel we're going to create chances and score. Few of them have actually ever played national league football. They have an average of 21 and that just shows how confident this side is."
As O-League contenders, one would have expected the hosts to hold on to the 3-3 to qualify.
Lovemore and Billy Scott combined for the former to make it 1-0. But they conceded a soft equaliser a few minutes later when a clearance shot smashed into Wellington's Tim Myers.
Tinsley then made it 2-1 from the spot in the 27th minute with little doubt about the infringement, although it seemed to have occurred just outside the box.
Before the second half, a deft Luis Corrales' cross from near the goal line found Ian Hogg's head for a straightforward goal, 2-2, with the Bay defence missing and showing lapses into the second spell.
Nevertheless, four minutes after the interval, Lovemore missed in the box but Barbero didn't, pushing it past goalkeeper Michael O'Keefe, 3-2.
Ex-Bay skipper Bill Robertson equalised 3-3 from a shot which had rebounded. With such a script, there was only one ending: Halpin upended, Tinsley converts, game over.
Wellington coach Matt Calcott was less inclined to talk: "What you wanna talk about ... Celtic FC? You know how I feel - disappointed to say the least. But if you lose four goals in a match, and five overall, you deserve to lose. What more can be said?" Calcott said.
Asked about the two penalties, he gave a look of "don't even go there".
Bay United face a David-vs-Goliath grand final away this Sunday against Auckland City.
The World Club Cup third placegetters despatched cross-town rivals Waitakere Utd 5-1 yesterday.
