Taranaki's Tom Butland incurred an interference penalty that ultimately would cost him a victory.
Despite this, Butland fought back well out of a combination situation.
Robinson improved on his impressive showing in the heats to score a splendid 13.87, which heaped pressure on Butland.
To Butland's credit, he posted a near-perfect 9.67 in his run, but fell on his final wave where he required an 8.42 to win.
The fall gave Robinson a well-deserved victory and bumped him up the rankings.
The Raglan pairing of Josef Jungwirth and Navryn Malone finished third and fourth respectively.
Robinson will be back in action this weekend at the Rip Curl GromSearch in New Plymouth, followed by another event at Mount Maunganui a week later.
Meanwhile fellow Northlander Billie Scott was unable to replicate her success from the first round of the Billabong Grom Series.
After finishing runner-up in Whangamata, Scott missed out on a place in the under-20 girls' final after placing third in her semifinal.