He birdied five of the opening seven holes and added birdies on the 12th, 14th and 18th holes in the best round shot by an amateur on the Bay's championship course in the club's 127-year history.
It equalled the course record set by Hawke's Bay professional Pieter Zwart at the Poverty Bay Pro-Am in March 2013.
Mountcastle, who won the 2018 New Zealand strokeplay and was beaten in the matchplay final that same year, has another chance to claim the title, but faced a tough opponent this morning in defending champion Sam Jones.
“What will happen will happen,” Mountcastle told NZ Golf. “Sometimes you meet people on a good day, and that's OK. Hopefully my golf can match or be better than theirs, and I get the job done.”
Kobori and fellow NZ Golf National Development Programme Mako Thompson (Maraenui) both shot 66s yesterday.
Kobori, the brother of women's top qualifier Momoka Kobori, had seven birdies while Thompson started with an eagle on the first and was 8-under after 12 only to double bogey the 13th.
Brettkelly added 69 to his pace-setting first round of 65.
Palmerston North's Tyler Wood was fourth on his own with a 9-under total of 135 (67, 68), while Thompson was one of four players on 6-under.
Kahu Tataurangi (Remuera), son of former PGA Tour pro Phil Tataurangi, had two eagles (the first and the 12th holes) in his second-round 68.
The cut-off for the top 32 was 2-over 146. Among those on that score was Rotorua's James Tauariki, who won the 2018 King of the Coast men's open at Tolaga Bay.
There were 18 sub-par rounds among the men yesterday, compared with 26 on the first day.
Overall, the par-4 10th hole played the easiest at an average of 0.39 under par while the par-4 16th was the hardest at 0.34 over par.
The 10th yielded the most birdies — 63 — while the 17th had the fewest — eight.
Cantabrian Momoka Kobori cemented the No.1 qualifying spot with a second consecutive sub-par round in the women's championship yesterday.
Kobori (Rangiora) added 2-under 70 to her supreme opening 68 for a 6-under 138 total.
She finished one shot ahead of Whanganui's Tara Raj, whose bogey-free 69 yesterday was the low round of the day.
There was distance back to third qualifier Vivian Lu (Royal Auckland and Grange) on even-par 144 (74, 70).
Kobori, however, faced a daunting prospect in her opening matchplay round this morning against 16th and bottom qualifier Darae Chung (Shandon), one of the form players heading into the amateur.
“I'm actually really looking forward to it,” Kobori told NZ Golf after her round.
“She's a great player and one of the top-ranked players in the country and I'm looking forward to testing myself against her.”
The cut-off for the women was Chung's +11 (78, 77).
The women played a modified par-72 course.
Stat showed the par-5 fifth hole was the easiest at an average of 0.20 under par while the par-4 16th was the hardest, playing 0.80 over par.
The praise continued to flow for the condition of the Awapuni Links course.
One player told a volunteer that he had played in the Taranaki Open at the New Plymouth course and it was in great nick, but the Poverty Bay course was “unbelievably good”.
