The Trailblazers regained the lead just before halftime through hooker Manu Anderson finding a gap on the fringes but, as they had a smaller reserve bench than Whanganui on a warm day, the hosts took control after the restart.
Lock Akosita Marr dummied into a gap and took off for a long-range try and, after kick-chase pressure created a penalty, reserve Mihipeka McKenzie-Mason quick tapped and fended her way under the posts, converted by reserve back Alyse Bird for 24-17.
Whanganui flanker Samara Pahl-Long scored from a close-range maul with King Country defending a player down due to on-field injury, and then Walker got her double after running back a clearance and combining with flanker Te Huinga Chambers to beat the last defenders.
King Country scored a fine consolation try on full-time as Boynton-Rameka attacked the corner and offloaded for Anderson to get her double.
Whanganui will rematch the Trailblazers in September for the fourth round of the Heartland Series.
“I would imagine they will only get stronger, more accurate, they’ve got some great pieces out there,” coach Junior Nepia said.
“Very happy with a lot of things, very happy seeing some individuals want to push themselves and have a go at it – happy no injuries.
“The goal was to get everyone rugby. We spoke about it as a group, trying to work on things.
“Some worked, a lot didn’t, but it puts us in a great position in terms of where we know where we’re at now.”
Whanganui 34 (B Walker 2, E Haitana, A Marr, M McKenzie-Mason, S Pahl-Long tries; A Martin con, A Bird con) bt King Country 22 (M Anderson 2, K Boynton-Rameka, L Law tries; J Hoet con). HT: 17-12 King Country.