In the chasing pack was current Northland No1 Kadin Neho (70) with a further shot back to Kerikeri's Alex Neely (71) and long-time Northland representative Bob Shirley (71).
A stiff breeze made scoring difficult in the afternoon, Saali Herewini, of Whangarei, the only player to go under par with a round of 71 moving him into second place.
Kerikeri junior Taylor Gill and Scott Wilson, of Waitangi, were the next best with afternoon rounds of even-par 72 that moved them inside the top 10.
Importantly though, McKeown followed up his hot start with a respectable 73 in the tricky conditions to extend his lead and claim a comfortable six-shot win.
With the final senior trial for 2014 now complete selectors will now have some tough decisions to make, particularly when it comes to the playing order of the side.
The eight-man team to contest the Garrard Shield against Auckland, Bay of Plenty and Waikato at Kerikeri from October 17-19 is due to be confirmed in the next week.
Top 20 at Waitangi:
1st: Chase McKeown (Waipu), 139.
2nd: Saali Herewini (Whangarei), 145.
3rd equal: Bob Shirley (Carrington), 146; Alex Neely (Kerikeri), 146; Kadin Neho (Whangarei), 146.
6th: Luke Brown (The Pines), 147.
7th: Taylor Gill (Kerikeri), 149.
8th equal: Scott Wilson (Waitangi), 150; Sheldon Kearns (Waipu), 150; Braden Keown (Waipu), 150.
11th: Julian Fowler (Northland), 152.
12th: Peter Bone (Whangarei), 153; Michael Perrin (Kerikeri), 153.
14th: Lee Neumann (Northland), 154.
15th: Josh Allen (South Head), 155.
16th equal: Jamie Thomas (Wellsford), 158; Jackson Arkle (Northland), 158; Iain Power (Northland), 158; Christian Nitsche (Whangarei), 158.
20th equal: Richard Poole (Kerikeri), 159; Ben Bowmar (Whangarei), 159.