The 35 men and women were, on the face of it, volunteers for art, though they each had their explanations.
"Just doing things," said a woman, who did not want to be named.
Another explained: "I kind of like getting naked. Usually it's in the bathroom, but I thought I'd take it a bit further."
Curiosity swept over the surfers squeezed into wet suits, and even, apparently, a big brown dog that lingered in a pool of water, naked himself though covered in hair.
The group had arrived about 9am and walked across to O'Neill Bay for some privacy.
They walked mostly in pairs and were mostly European, and began the day dressed mostly casually - but had little else in common in terms of appearance.
When they undressed, the unusual sight of 35 strangers clustered together appeared almost desolate in its bareness.
The only familiar imagery of such a group is of refugee camps - and, indeed, that is the aesthetic of the work that last year won Trinh international photographer of the year for Black+White Photography magazine.
"It's an intimate thing," Trinh said of the naked body.
"And it can be beautiful. For people who have never done it before it becomes a chance for personal discovery - because most people are self-conscious, and it's a chance to come to appreciate your own body."
The group dipped into the cold water to plenty of laughter - and began striking poses around the rock pools. But Trinh asked that they lie down as much as possible, and they became a mass of half-submerged flesh.
"The reality is we put [clothes] on in the first place," said a man afterwards, "because it was cold."
His idea was that nudity was a natural state. But the nakedness at Bethells failed to give any immediate impression of being indigenous to the landscape, or to wilderness.
The bodies were very pale to brown, with lots of floppy bits.
"I know that in the past, particularly females have said they never felt more confident in their lives, because they were suddenly surrounded by people who were the same as them. There's no perfect body out there, and it takes this experience to realise that," Trinh said.
There was a lot of unnecessary censuring of nudity, he said.
During 90 minutes of shooting, the men and women clambered over rocks in slow movements, taking on a languor. Their skin became marked from sitting on sharp rocks and darkened by the black sand.
And by the end, it was the blue collar of Trinh's polo shirt that stood out most from the scene of brown rocks, grey water and plain skin.
