They switched to more grinding tactics - keeping the ball in tight around the ruck - during the second half, but the Dolphins' defence was up to the task right until the final 10 minutes when both teams scored in the far corners and Metro were at least denied their bonus point.
"We got a lesson in wet weather rugby and handling let us down," said Metro coach Carl Gibson. "Just pushing passes early so that was the call at halftime - not push passes and carry better."
The Dolphins' back three showed good composure, securing three of their team's four tries by handling the key passes when they counted.
For Metro, flanker Corbin Warren was very busy although he did suffer referee Noah Viliamu's ire when coming through the ruck, while tryscoring hooker Harry Black and prop Keanu Puki-Kaa refused to quit.
Halfback Riley Culver did his best while unfortunately, star fullback Te Rangatira Waitokia could only come on at halftime, as Metro were trying to protect his niggling injury.
The home side scored first shortly after the forwards pushed the Dolphins off their scrum ball to win the feed, with the ball spread wide for Black to force his way over.
However, mistakes and penalties by Metro in their own half let the Dolphins strike back immediately with two tries to the outside backs, creating overlaps in the far college building corner for 10-5.
They went on to bag another try in the opposite corner from a backline spread right on halftime, with the near sideline conversion hurting for 17-5.
The scores would be locked there as the rain stayed heavy in the second half, with Metro twice grinding their way to the line, only to lose the ball or play it off their feet for penalties in the face of solid defence.
The Dolphins finally cracked with five minutes to go as Metro reserve winger Owen Clay swept up a loose pass and dove over in the corner for 17-10. But with the rain finally stopping, Dolphins showed composure and worked their way to Metro's far corner, where after more than 10 phases right on the chalk, they finally snuck across to grab a bonus-point try.