Despite the southerly wind beginning to strengthen, Munn also birdied the 14th, 15th and 16th to be six under at the turn.
Was he thinking about breaking his record round? "I tried not to think about it but I was going for it."
His second half was more of a battle but he didn't drop a shot and he birdied his final hole when he finessed a sand wedge from a rough lie to 4.2m and sank the putt for 65.
"I love it round here," he said. "It must be the beach. I just love the greens."
The young amateur was not getting ahead of himself and thinking of another tournament win.
"I'm not looking that far ahead. I hit a couple of bad shots and I'll hit the range this afternoon."
Munn earned the admiration of his playing partner, 2010 winner Mike Hendry, who shot a first round par 72.
"He was awesome. He could have birdied every hole," he said.
"He putted fantastic and it's good to see a young guy play as well as he did."
Hendry, who heads for two rich tournaments on the OneAsia Tour next week, was not so happy with his own game.
"All in all it was pretty poor. I struggled to read the greens. But there's plenty more to be played and I think I'll be there at the finish."
The 18-year-old An had an up-and-down round with eight birdies and four bogeys in his 68.
He never got below four under and needed a birdie on the short 17th to get back to that number.
Moorhead, playing fulltime now after a spell at the New Plymouth club, would have been much closer to Munn but for misadventures on his last two holes where he dropped shots after misjudging putts.
Fox, in his first season as a professional, birdied his last two holes after a steady round played in the windier afternoon conditions.
The 15-year-old Ko, who won this tournament last year despite Phillis Meti setting the course record of 69, dropped only one shot when she was short with a wedge to the 16th and then too strong with her chip.
She was particularly pleased with her putting.
"I holed a few long putts and that doesn't usually happen for me," she said. She admitted to starting nervously and profiting from some lucky bounces.
The Ask Metro tournament continues with 18 holes today with a cut of the top 60 and ties for the final 36 holes at the weekend.