Halfway through the second half the Pioneers had stretched their lead to 40-4, with Koroi's partnership with Wiremu Snowden-Te Iringa a key factor.
Snowden-Te Iringa notched a hat-trick, with Koroi dotting down and converting six tries for a personal tally of 16 points.
In the last 20 minutes, the Bulls gained some respect with three late tries, to end the match 40-20.
The Pioneers progress to Saturday's major semifinals and will face the Hikurangi Stags.
Saturday's second game featured big improvers Otangarei Knights against Portland Panthers.
Otangarei winger Dane Tito scored first in the seventh minute, Portland hitting back through skipper Robert Nathan in the 11th minute.
This spurred the Panthers pack - led by the Salase brothers, Michael and Alex - into action, forcing the Knights into desperate defence.
A Panthers turnover on a third consecutive set of six resulted in Knights fullback Zion Rota galloping 70m to score.
The Knights went into halftime 14-6 up, after standoff Jordan Katene had scored against the run of play.
In the second half, the momentum kept swinging from one team to the other. First, the Panthers charged back into contention with Alex Salase grabbing a well-earned try. Then Sharif Mangu scored in the 55th minute for a 16-14 Panthers lead.
However, two penalties put the Knights in front 18-16 until the 78th minute.
Then with the Knights on attack, Linn Hunapo looked certain to score as he stepped inside the last defender, who flung out a reactionary arm, felling him.
Play was halted while Hunapo was carried off and taken to hospital for observation.
When the game resumed the Panthers' Michael Salase was sent off and the Knights receiving a penalty try to win 24-16.
They play Takahiwai Warriors at 3pm on Saturday at Toll Stadium.
Major semifinals
August 2 at Toll Stadium
Hokianga Pioneers v Hikurangi Stags, 1pm
Otangarei Knights v Takahiwai Warriors, 3pm
