It was an agonising loss for Collegiate, who made the final and lost 33-7 to the Reuben Thorne-coached Christs College from Christchurch last year.
They made the final for a conseuctive year by impressively beating the hosts, Wellington College, 32-5 on Monday.
Nelson last won the tournament in 2017 and their win avenged a 19-13 loss against Collegiate in the opening game of 2018.
Playing in Wellington, Nelson opened the scoring for the final with a penalty goal early before Collegiate responded through a try scored by Hunter Morrison.
The try was converted by Te Atawhai Mason despite a strong wind sweeping across the field and Collegiate were rolling with a 7-3 lead.
But that was the last time they would score, Nelson replying with one of their own in the 24th minute and holding onto a one point 8-7 advantage until half time.
After halftime the weather took a turn and the rain came falling down, but that did not slow the southern men down.
Nelson scored another try, but missed the conversion leaving Collegiate with a chance to complete a comeback of their own.
But their 15 minute attacking assault was defended well, meaning Collegiate will have to leave it until next year to try and break their 18-year drought.