A Luke Brattan thunderbolt in the dying stages of the match proved the difference as the Roar ran out 2-1 winners against a side which the Brisbane coach picked for a top four finish.
Mulvey also made it clear he was unhappy with Melbourne Victory's handling of the announcement of Besart Berisha's end-of-season transfer between the clubs; news which proved a distraction in the build up to the Phoenix game.
Wellington had controlled the early stages of the game by holding firm defensively and playing some crisp attacking football going forward, but after they scored through Stein Huysegems, Brisbane upped their own tempo and worked their way back into the match.
The equalising goal came from 18-year-old debutant Devante Clut, who showed plenty of ability and confidence in his first game, before Brattan sealed the deal in spectacular fashion three minutes from the end.
"Let me tell you this; Wellington Phoenix are top four material, and we've beaten them three times this year," Mulvey said.
"That's the magnitude of this result.
"They've been playing ever so well lately, and Ernie's got them really aggressive.
"And they gave us some problems, they could have been two up at half time.
"But then we came back into just towards the end of the first half.
Everything fell into place for Brisbane in a thrilling second term, according to Mulvey.
"I just loved that second half, it was fantastic," he said.
"That's what football's about for me.
"We went for it, I thought we played very well, we got the goals and the crowd got involved, brilliant stuff."
As for the impressive debut of playmaker Clut, Mulvey joked that he may soon be on his way overseas like the recently departed Kwame Yeboah, who left Brisbane after just a handful of games.
"I believe there's an offer from Borussia Monchengladbach," Mulvey quipped when questioned about Clut's performance.
"No, look, I was really pleased for Devante, he was blowing the big ones after about 65 minutes, but we got him off just in time.
"But absolutely delighted for the boy because he's trained with us probably since July with the first team and he worked his socks off inmidfield.
"I told him (he would be starting) about an hour and a half before kick-off.
"I thought that would probably be the best thing to do for him, and then he did the rest."
Reiterating comments he had made pre-match, Mulvey was also critical of Victory's decision to go public with the signing of Berisha mid-week.
"It's been a bit of a circus this week and I didn't appreciate the fact that it was in the public domain on Wednesday because we had a game to prepare for on the Friday.
"So I didn't think there was much respect shown to us."
