Bethlehem also conceded too many penalties in the first half, one of their players yelling "these penalties are killing us," after the fourth straight penalty cost them three more points before the break.
In the second 40 minutes the action was more like the Bethlehem team that came third in the first round of the competition.
A charge for the line by hooker Scott Crowley opened the scoring for Bethlehem then Chris Johnston scored. The replacement fullback took off through a gap on halfway and backed himself all the way to the try line. Gerlach converted to close the gap to 15-14 before a late tackle led to a yellow card and a chance for Gerlach to win the game in injury time. He did not disappoint.
Bethlehem coach Grant McCoid was thrilled. "The boys made a lot of errors so based on that they could play in our half and they didn't take much to go over our try line. Otherwise, I thought the boys played really well except for the handling errors really. The fightback was awesome. You don't get to come back from 15-0 that often and just scrape home ... Hopefully that is a defining moment for the boys to move forward and believe in themselves a bit more."
McCoid said they had been building towards this final championship round and were not focused on results in the first round, despite finishing third behind Otumoetai College and Tauranga Boys' seconds.
"What we wanted to do in the first round was performance based, rather than the scoreboard, and we didn't really improve as we could as a team so today was a way better step in the direction we want to go."
Bethlehem College 17 (Scott Crowley, Chris Johnston tries; Cam Gerlach 2 cons, pen) Opotiki College 15 (Sonny Collier, Mike Stevens tries; Kaleb Maxwell con, pen). Halftime 15-0 Opotiki.