"I thought I'd just roll it up, and it went in the hole. It was my longest putt of the year," he said with a slightly bemused air last night.
The eagle - two under par - left him at seven-under 64, two off the single-round Open record at the course, set two years ago by Australian Paul Gow.
Gardiner, a frequent visitor to New Zealand since coming for the Claire Higson Trophy under-23 transtasman contest in 1993, had a steady first nine holes in one under.
But he chewed up the tough Paraparaumu Beach links course in spectacular style on the homeward nine.
He nailed birdies at 11, 12, 15 and 16 before his eagle finish to be at the top of a leaderboard made up entirely of Australians and New Zealanders - not an American in sight.
Tiger Woods had a steady one-under 70 yesterday, which left him feeling happy enough, despite battling to sink his putts. There was the occasional spectacular shot, some brilliant saves from dodgy spots, and the clear impression that the greens offer Woods a decent challenge.
"They're definitely difficult to read. I felt I should have been a few shots lower, but you don't want to shoot yourself out of the tournament," he said.
The movement at the top of the leaderboard did not really start until late in the day. For a large chunk of proceedings, Victorian Craig Spence and New Zealander Stephen Scahill shared top at four under.
Then, with a rush, West Australian Brett Rumford moved up, followed by McLean and Hamilton professional Steve Alker, before Gardiner's dramatic final flourish.
McLean, a 23-year-old from Wahgunyah, New South Wales, who now lives in Minnesota, took Gardiner's route by peeling off four successive birdies on the second nine.
"There's a long way to go, but if I play solid like I did today, anything could happen," he said.
Another with a confident spring in his step was Michael Campbell, who ended the day at four-under 67, nicely placed and with a gleam in his eye.
Three times he had putts lip out of the hole, and his new driver added, he estimated, 10 to 15 metres off the tee.
"Today was all about patience. This is a really tough course, but I'm pretty confident for the next three rounds," the world No 29 said.
He was not about to get carried away by Woods' steady but unspectacular round.
"You always expect Tiger to shoot six under. You can't count him out.
"If I beat Tiger by two or three shots at the end of the week I'll be doing pretty well."
Alker was steady, seasoned Australian Craig Parry readied himself for a serious tilt later in the tournament with a 67, and Aucklander Brad Heaven was one of three amateurs to shoot red figures, at one-under.
But the day belonged to Gardiner, who played the traditional role of surprise first-day leader, with a dry touch.
He conceded his preparation for the Open was "a bit rusty."
"I shot five over yesterday [in the pro-am] so my expectations weren't that high."
What about the wind? No problem for the laid-back 25-year-old from Terranora, New South Wales.
"I like wind. It requires patience and that's a good part of my game. I don't worry about which way the wind is blowing. I just get up, hit it and see where it takes me."
Woods? Which Woods?
"We're all capable of shooting 65s. It's a matter of doing it day in, day out."
The first day drew an official attendance of just over 10,000, fewer than organisers had been hoping for - 15,000 to 20,000 are needed to make the tournament sums work.
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