Horowhenua-Kapiti came back into the match with two penalties to Josh Hamilton but rarely threatened the Buller line with most of their attacking having to come from deep within their own territory and invariably meeting with resolute Buller defence before the tryline came into view.
Buller's second try again came from a Nabainavalu break, with William Saukuru the recipient of a fine ball which allowed him to score by the posts.
Horowhenua-Kapiti's lack of basic control saw Buller extend their lead straight from the second-half kickoff. Horowhenua-Kapiti failed to kick the ball the required 10m and, from the resultant scrum, Andrew Norton-Taylor scored in the corner.
Two Marty Banks penalties took the score out to 27-9 but, as the injury toll mounted and the Buller ranks began to resemble a war zone, it seemed Buller would be denied the try which would give them the bonus point, especially as the Horowhenua-Kapiti team seemed rejuvenated by replacement halfback Tainui Brown who scored an excellent solo try after 30 minutes.
Horowhenua-Kapiti flung the ball in all directions but the re-jigged Buller team held on and in the very last seconds a Horowhenua-Kapiti mistake allowed Banks to scoop up the ball and dash 50m without a hand being laid on him to score under the posts to give Buller the bonus point which could prove vital, given the closeness of the points table.
Buller had many heroes on the day with Saukuru and centre Trael Joss joining Nabainavalu as outstanding backs and Craig Moore and Isaiah Namua outstanding in the pack.
For Horowhenua-Kapiti, flanker Anthony Fox was the best of the pack and Tainui Brown a dynamic halfback in the second half.
The only sour note for Buller was their injuries and the fact they were on the wrong side of a lop-sided penalty count. Referee Michael Lash lived up to his name, awarding 22 penalties and free kicks to Horowhenua-Kapiti and only nine to Buller.
Buller 34 (Craig Moore, William Saukuru, Andrew Norton-Taylor, Marty Banks tries; Banks 4 cons, 2 pens) Horowhenua-Kapiti 16 (Tainui Brow try; Josh Hamilton 3 pens, con). HT: 14-9.