An hour earlier, they had been strangers enjoying a warm Sunday afternoon at a waterfall in the Brisbane Water National Park, near Gosford.

But a tragic turn of events at Somersby Falls left Nick Weston and a woman, named Debbie, in tears and hugging in grief as they tried to comprehend what had just unfolded before them.

Mr Weston, 26, had been visiting the falls with his mother, Ruth, when they heard a young man calling for help.

The 23-year-old man, who Ms Weston said was named Bradley, appeared to have lain down on his stomach to peer over the edge of the falls.

But, as he manoeuvred his body around to return to safety, he appeared to have slipped on moss and slid over the edge, Ms Weston said.

"He was trying to grab on to something, but he was gone that quickly," Ms Weston said.

The man plunged about 10 metres onto rocks and into the water below, in front of about 20 people.

Ms Weston said Nick "flew down over the rocks" to try to help Bradley, and was quickly joined by another man who also came to his aid.

"We were yelling out 'Don't move him', because that's what you're told, but they had to move him, otherwise he was going to drown," Ms Weston said.

"He was sort of going in and out of consciousness at first, and Nick was holding him up. I could hear Nick talking to him, trying to keep him conscious, and he was holding the towel around his head trying to stop the bleeding."

An off-duty paramedic and police officer also assisted, before emergency services arrived a short time later.

However, police said the man was unable to saved, and died at the scene.

Ms Weston said her son was holding Bradley throughout the ordeal.

Ms Weston said she was standing at the top of the falls when a woman named Debbie came over and asked if the injured man was all right.

"I said, 'I don't know, he doesn't look like he's in a good way.' I didn't realise I was speaking to his mum. Then I was trying to console her. It was a terrible, terrible thing. I could only try to imagine to put myself in her position."

Ms Weston said emergency service workers tried to resuscitate Bradley for an hour, before he was pronounced dead.

When Nick climbed back up to the top of the falls, he met the mother of the man whose life he had just tried to save. United in grief, they held hands and embraced.

"I just can't imagine the family's grief," Ms Weston said, adding that she and her son had been profoundly affected by the tragedy.

"You plan a nice day out, you don't expect a tragedy like that to happen. It's just shown how fragile life is and how precious, you know?"

Somersby Falls is a popular picnic spot, surrounded by thick rainforest. A 500-metre return walking track from the picnic area winds around the top and bottom waterfalls.

"The walk is steep and slippery in places, so keep an eye on the kids as they walk along the track," the National Parks NSW website says.

Instagram users regularly post photographs of people standing on the rocks under the waterfall.

Ms Weston said emergency service workers told her they had been called to the falls five times this year to treat injured people.

"People take their kids here to swim and things. You don't really swim, there's shallow puddles. Personally I think it should be blocked off, so you can go down and look at the falls but not necessarily walk around and swim," she said.

Another woman, named Natalia, said she was also at the falls at the time and praised the efforts of everyone who tried to save the victim.

"The work of everyone in rescue and emergency is beyond of words. we saw your work first hand and it left us speechless," she wrote online.

Police said the incident was not considered to be suspicious, and a report was being prepared for the coroner.

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