What transpired that fateful afternoon in November 1986 was to change many Canberra lives forever and create a political storm around the public safety of one of the capital's most popular swimming areas.

For many years, Casuarina Sands had a reputation for being a tricky place for swimming when the river was up and running.

A concrete weir right had been built right across the river in the 1960s, with the best of intentions, to create a pooled area in which to swim during the long, hot Canberra summers.

With the coarse riverbank sands providing a quasi beach-like environment and overhanging trees for shade, hundreds of people flocked to the river banks to cool off.

Mixing with his friends at a barbecue on that warm November afternoon, Colin Trinder had barely glanced at a woman and a boy who were carrying an inflatable rubber raft upstream.

It had been raining a little on and off in previous days so the river wasn't in flood but it was running strongly.

As many Casuarina Sands swimmers would attest, the river was deceptive in that even what appeared to be a sluggish current could exert a significant downstream drag.

A warning sign advised of the conditions. Swimming was prohibited at the location until October 1, and then the warning varied between "exercise care" and "unsafe".

"A head of water, it would have been more than half a metre of water, was flowing over the weir, all the way across it," Mr Trinder recalled. "So it was a quite a volume of water."

The rubber raft, with the boy and the woman in it, both wearing life jackets, paddled out and began to drift slowly towards the weir. In seconds, the pair was in trouble.

"I didn't actually pay much attention until we saw the raft go over the weir and then get trapped in the backwash behind [the weir]," Mr Trinder said.

"The strong back pressure created by the flow over the weir basically pinned the raft in place mid-river."

Pressure waves and eddies created by strong water flow around river obstacles have been harnessed by whitewater canoeists for years. Rushing over the weir, the water flow bounces off the rocky bottom and returns straight ack up, creating a standing wave.

The quickest extraction from a pressure wave is said to be counter-intuitive; instead of staying at the top of the water flow, it's close to the bottom where the upward pressure decreases and horizontal pressure becomes a more powerful controlling force.

But for 13-year-old Brandon McIlroy, now alone in the bobbing raft after his companion had fallen out and been carried downstream to the opposite shore, the swirling water behind the weir created a trap from which the raft and its small occupant could not escape.

"We grabbed a tow rope out of the car but even when we waded out into the river as far as we could, the rope was still about a metre too short for him to grab," Mr Trinder recalled.

Police were called and a rescue team arrived just after 4pm.

Standard swift-water rescue techniques suggest a line should be secured on the opposite bank but this wasn't an option, as the rope kept getting dragged over the weir.

Serious concerns were held for the safety of the boy in the raft as the water was cold and the raft was slowly deflating.

Police officer Tony Lewis then took the brave decision to put on a harness roped back to his team on shore, and swim out.

His plan was bold but simple: to drift down slowly with the current, float over the weir, grab the boy and have his team drag them back to the bank.

Mr Trinder positioned himself below the weir on a log sticking out into the water, waiting in case the boy fell from the raft.

With the police fully occupied above the weir, he reasoned that should the boy fall out of the raft he could swim out and grab him.

An excerpt from the police magazine Platypus then takes up the story:

"When Senior Constable Lewis got within three metres of the weir, he was suddenly forced toward the near bank and went over the weir wall, about four metres from the boy."

Mr Trinder, watching from below the weir, said that the police team upstream couldn't see what was happening and were trying to drag the officer back up as tonnes of water crashed down on him.

"He [the police officer] was getting physically crushed by the water and the pressure of the harness. He couldn't release himself and it was holding him under water," he said.

"We were shouting to the police to let the rope go. They couldn't hear us over the roar of the water but eventually they let go and the officer was washed over the weir and went under.

"I saw his face under the water and saw he'd been spat out of the pressure wave, so I dived in and held his head up.

"We were both tangled up in the rope but luckily we were both carried closer to the river bank by that time and I could swim him in to shore."

The officer was injured and unconscious, but breathing.

As an ambulance was called, repeated desperate attempts were made by police and the public to get out to the boy still trapped in the raft.

Tragically, an hour or so later the raft slowly deflated, the boy slipped under the water, and disappeared. His body was found the next day.

"It was an awful situation and I suspect he [the boy] had been affected by hypothermia. We tried all we could to reach him but we just couldn't," he said.

"I've always wondered if we had tried some other thing, some other way, linked arms and waded out, or somehow got a rope to him earlier. It's a regret I've carried for a long time."

Colin Trinder says he still reflects on the incident from time to time.

"It's difficult to be that physically close to someone in real trouble and unable to save them.

"I'm quite a strong swimmer and I now wonder if I had tried to swim out in my jeans and T-shirt, would I have got to him? I don't know, most probably not; there's a fine line between bravery and stupidity."

He didn't hesitate to jump in to save Tony Lewis. Now he wishes he had tried to do the same for Brandon McIlroy.

"But it was brave thing for the police officer to do. I don't think I would have volunteered to go over the weir with a rope around me like he did."

Senior Constable Tony Lewis, now retired, received a bravery award for his actions that day. Colin Trinder received a letter commanding his actions.

As the river rolls on and visitors to Casuarina Sands come and go, the dangers of our inland waterways are masked by their seemingly peaceful and benign appearance. Tragically, it's a mistake that some have paid for with their lives.