Koh stormed home in sensational fashion on the Remarkables course to card 17-under-par after the opening two rounds at Millbrook.
The 25-year-old equalled the Remarkables course record when he finished with a score of 10-under-par 61 yesterday.
"It’s very meaningful.
"I didn’t even know I shot the course record," Koh said.
"I didn’t even look at the scores. I am really proud of myself."
He finished with nine birdies and an eagle and was particularly pleased with how he struck his irons, which helped him attack the greens.
"For the next two rounds I will focus on positive thinking.
"I played very well ... I want to carry this momentum into the weekend."
James Marchesani was also red hot through the morning session.
The Victorian added eight birdies to his tally, and after a bogey on the 13th, he bounced back immediately with an eagle on the par-5 14th hole.
He was comfortable with his performance, hitting a bunch of great shots to get him on the green.
"I’ve been putting great for what feels like that last six to eight months," Marchesani said.
"If you’re hitting it and giving yourself plenty of looks — like [Thursday] I hit 18 greens — if you’ve got 18 birdie putts then a lot of them are going to go in.
"I knew if I could get a lot of looks, the hole’s going to start looking pretty big, which it has."
Millbrook provided a great atmosphere — thanks in part to some balmy conditions — and Marchesani expected that to continue through the weekend.
"I think it’s probably the busiest I’ve seen it, especially on the Thursday and Friday.
"It’s a good buzz. There’s a good vibe.
"I think the change in the course across the weekend will see a lot of spectators as well — it’s been a lot of fun."
Michael Hendry and Josh Geary lead the Kiwi contingent, finishing on 10-under-par.
Kiwi Daniel Hillier is close behind on 9-under-par, followed by Ben Campbell (8-under-par), Denzel Ieremia (7-under-par), Mark Brown (6-under-par), James Tauriki, Sam Jones, and Steven Alker (all 5-under-par).
Any golfer deserves the $2 million purse, but everyone — including tournament director Michael Glading — would love to see a New Zealander lift the Brodie Breeze Trophy tomorrow night.
After high scores were posted at Millbrook, the cut began at 5-under-par.
Kiwi Danny Lee, who had a small army following him all day and was often held up by the group in front, missed the cut, finishing 2-over-par in his first appearance at the Open in 16 years. Kiwi Nick Voke also missed the cut.
Defending champion Takahiro Hataji missed out after finishing par and former Open champion Australian Brad Kennedy missed out as well.
NZ Open
The scores
-17 Guntaek Koh (Korea)
-14 James Marchesani (Aust)
-13 Bobby Bai (China)
-11 Ryan Peake (Aust), John Catlin (US), Ian Snyman (South Africa)
-10 Michael Hendry (NZ), Elvis Smylie (Aust), Ryosuke Kinoshita (Japan), Justin Warren (Aust), Josh Geary (NZ).
