The rampaging 135kg prop barged over for two first half tries and added a crucial second half try which effectively sealed the outcome.
Asi was virtually unstoppable from two or three metres out and coach Ryan Martin said he was a special player. ''He's one in a million in terms of what is around in New Zealand.''
But centre Hemaua Samasoni was also impressive for the home side. He ran straight and hard and was a colossus on defence. No8 Slade McDowall was busy and combative.
St Bede's College beat Shirley Boys' High School 32 13 in the Canterbury final and will have to make the trip south to play OBHS in the South Island final at Littlebourne. Kickoff is at 2.30pm on Saturday.
Martin said St Bede's had an ''industrious forward pack'' and it was well structured with a quality kicking game.
''We will need to make sure our defence is really strong,'' he said.
Whether St Bede's has anyone who can stop Asi remains to be seen.
He made an early impact against Southland Boys', running 20m and swatting aside four of five would be tacklers. But halfback Tim Hogan helped created the game's first try. He made a decisive dart before Asi finished off the move when he rumbled over.
The tighthead prop crashed over again after second five eighth Kabe Calder was held up.
SBHS trailed 20-6 at the break and the hole got deeper when Calder got on the outside of his opponent and made an angled run towards the corner to score.
That try went unconverted and the visitors responded with two converted tries to put the game back in the balance. Lock Tim Young spun his way out of a tackle to score and hooker Flynn Thomas caught the home team napping when he dived over following a quick tap.
Southland Boys' had enjoyed a productive 15min period but Asi clinched the winner when he barrelled over from close range.
South Otago High School beat Gore High School 45-5 in the regional final of the co educational tournament in Gore on Saturday and will play St Andrew's College in the South Island final in Christchurch on Saturday.
OBHS v SBHS
The scores
OBHS 32
Sione Asi 3, Jordan McEntee, Kabe Calder tries; Josh Buchan 2 con, pen
SBHS 20
Tim Young, Flynn Thomas tries; Pedro Bezanilla 2 con, 2 pen.
Halftime: OBHS 20-6.
