The 26-year-old, who had missed four previous shots at goal, had ice in his veins to land the pressure kick with a howling gale behind his back in front of a packed Auckland crowd on Saturday.
Broncos fullback Reece Walsh sent the kick-off to start golden point over the dead-ball line to concede the penalty.
The Warriors were without three of their injured stars in James Fisher-Harris, Charnze Nicoll-Klokstad and Roger Tuivasa-Sheck. After last week's 42-14 thrashing by Melbourne it was a stellar response.
The hosts deserved to win and were clearly the better side. Middle forwards Jackson Ford, Mitch Barnett and Erin Clark were enormous, while 19-year-old backrower Leka Halasima announced himself as a star of the future in his first starting match in the NRL.
Broncos coach Michael Maguire picked three hookers in the 17 - Billy Walters, Cory Paix and Tyson Smoothy - but it backfired.
Props Payne Haas and Pat Carrigan were left with too much to do, with the Broncos one big bopper short.
Haas's second stint was all power and got the Broncos back in it, but it was flashes of brilliance from Adam Reynolds and Walsh rather than an all-round team effort that flattered the visitors on the scoreboard.
When towering Warriors back-rower Marata Niukore scored in the 64th minute for an 18-6 lead it appeared to secure the hosts the match, but Metcalf missed a simple conversion.
Walsh, who injured a knee in the opening minute, found a brilliant cutout pass to put winger Jesse Arthars over, and into a howling gale from the sideline Reynolds somehow managed to convert.
Then Reynolds put in a left-foot kick, Ed Kosi missed his chance to ground the ball and Walsh put his hand on it. Reynolds again landed the conversion from out wide and it was 18-18 with seven minutes remaining.
The Broncos did not make the most of the wind in the first half.
An early try to Walters was countered by one to Halasima after a bad missed tackle by Jordan Riki.
After the break sensational fullback Taine Tuaupiki kicked a loose pass in an opportunist play on the last tackle for Kosi to give the hosts a 10-6 lead in the 51st minute.
Nine minutes later centre Kurt Capewell swooped on another poor pass by Ben Hunt and raced 50m before getting a one-handed pass to Metcalf to score.
The closing moments were drama-charged, but Metcalf was a worthy hero.
Tuaupiki ran for 174m from 23 brave runs to confirm his status as one of the game's most exciting little men.
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