In a match that see-sawed between the two sides, Brisbane took the lead through Dimitri Petratos, only to fall 2-1 down.
Michael McGlinchey and Roy Krishna found the back of the net for the home side.
But, Roar fought its way back into the contest and found an equaliser when, on the hour, Jade North scored.
Former Sydney FC striker Powell, who joined Wellington Phoenix after a wonder-season at APIA Leichhardt Tigers in the NSW NPL, came off the bench to tap home the winner with ten minutes to go.
With the Phoenix's A-League license set to expire next year, and its future still under negotiation, Ernie Merrick's men showed no lack of intent against the three-time champion Brisbane.
However, with the Roar defence pressing high, the home team struggled to find rhythm or flow.
Petratos and Jerome Polenz were proving particularly troublesome down the right, but although Roar worked hard to force a series of corners Wellington's makeshift defence scrambled clear.
The absence of injured skipper Andrew Durante left it vulnerable on the counterattack, with the vision and pace of Thomas Broich proving key in setting up Australian Under-23 rep Petratos for a lethal left-foot finish in the 26th minute.
Phoenix pushed forward, with stand-in Roar keeper Jamie Young doing well to deny first Jeffrey Sarpong from 10m out then Roly Bonevacia from close range as the half-hour mark ticked over.
McGlinchey had no such problems from a 35th minute free kick just outside the box, curling his right-foot strike around the wall and dipping it perfectly into the bottom left corner.
Wellington took less than five minutes from the restart to snatch back the lead, Krishna slamming home his second goal of the season after a snappy one-two with Sarpong.
Brisbane continued to threaten from set play, former Phoenix defender Jade North volleying home from their 10th corner of the match in the 60th minute to again level the scores.
With the game drifting towards a draw, super sub Powell galvanised the Phoenix when he found himself unmarked on the far post and tapped home the winner from a teasing Tom Doyle cross.
Wellington Phoenix 3 (McGlinchey 36', Krishna 49', Powell 80')Brisbane Roar 2 (Petratos 26', North 60')Westpac Stadium, WellingtonReferee: Jared Gillet
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