COB was kept on the defensive but managed to get themselves out of trouble until a penalty kick awarded to Waikanae saw them slip further ahead at 6-11.
Undaunted, and with four minutes left until the end of the first half, COB turned the tables and worked the Waikanae five metre line methodically to find the gap and take their reward to draw 11-all.
Within eight minutes of the second half COB took a penalty kick and the lead for one last time, at 14-11.
For the next 15 minutes both sides were evenly matched, as attack turned to defence and back to attack.
COB missed a penalty kick and then Waikanae had the ball, running the ball down the paddock followed with a kick through to help set up a second try and take back the lead, 14-16.
Despite a push back from COB, Waikanae saw their opportunity and try number three was nailed in the 28th minute to stretch their lead 14-23.
An unlucky bounce for COB saw a line out in their 5m area and Waikanae worked their magic across the try line again, to run the ball neatly into the corner for an unassailable 14-30 lead with just four minutes left on the clock.
COB gave it their all and came achingly close to scoring a second try but the Waikanae defence held until the final whistle.
COB try scorer was A. Neilson and penalty kicks taken by S. Solomona.
Waikanae try scorers were R. Hunter, J. Hinds, K. Milligan and B. Mear; conversions and penalties taken by E. Reti.
COB assistant coach Ricky Fonoti said the loss was hugely disappointing for the boys who were "pretty hurt" by the score.
"We prepared well and were confident coming into the game. We competed well and had the energy and intensity for the semifinal, it was just little mistakes and Waikanae punished our mistakes.
"It was a tough game, Waikanae is a quality side."
Waikanae coach Simon Fifield said the teams were fairly evenly matched in the first half but they had the belief they could win.
"We didn't finish the opportunities in the first half but held [them] in the second half and scored tries."
He said COB had been the team to beat all year and Waikanae was enjoying the moment but it would be "back on the horse next week" when they face five time defending champions Paraparaumu who beat Shannon by one point, 21-20.
Senior Reserve semifinals results:
At Shannon Domain: Wanderers 24 v Shannon 10.
At Levin Domain: COB 17 v Rahui 8.
HKRFU club finals next Saturday:
Waikanae v Paraparaumu in the Premier final, kick off 2.45pm Levin Domain.
COB v Wanderers in the Senior Reserve final, kick off 12.45pm Levin Domain.