On that fateful day, an extended family drove up Red Hill to look at the city lights. They could see a storm brewing over the distant hills, so headed home.

Then the storm broke.

Within an hour, seven young people had lost their lives in the flash flooding caused by the severe thunderstorm and torrential rain. All seven were aged under 21, with four under 18.

The victims were Smith siblings Carmel (19), Margaret (15) and Michael (6), from Lyons; their Seymour cousins Jennifer (12) and Dianne (8), from Wangaratta; and Canberra locals Lon Cumberland (18) and Roderick Simon (20).

The extended family was split between two cars. Parents Ben and Mona Smith, their eldest son Peter, and relatives Kevin and Noreen Seymour were in the first car. There was already water over the road when they drove across the Yarra Glen intersection with Melrose and Yamba drives. The Smith children and the Seymour girls followed in the second car.

Ben turned around to check they were still behind him, and was horrified to see the intersection disappear beneath a wall of fast-flowing water. The children managed to climb out of the car and hang on to the guard rail. Kevin and Peter jumped into the water to rescue them and were knocked off their feet by the strong current but managed to cling to a light pole. They could only watch helplessly as their family was swept away by the torrent. Bystanders restrained Ben from rushing into the water.

Lon Cumberland, who was partially disabled, was driving home to Braddon from a game of wheelchair basketball in Phillip. His car was known to be unreliable in wet weather and may have stalled on the flooded intersection. Lon became trapped as the floodwaters surged around his vehicle and could not escape.

Rod Simon had just delivered his infant son to the baby's grandmother and was on his way to collect his wife from Farrer. He never arrived. His car was swept off Yamba Drive, south of its intersection with Hindmarsh Drive, and into the Long Gully Creek drainage channel.

Nearby Curtin residents heard the cries of the people stranded in the floodwaters and the shouts of those trying to rescue them, while eyewitnesses to the unfolding tragedy reported waves of about half a metre crash over the intersection. Cars were seen floating down the drainage channel.

Police and bystanders formed human chains to try to save as many as possible. Even the rescuers were in trouble; Constable Jeff Brown was swept downstream by the strong current while trying to reach Kevin and Peter, still clinging to the light pole.

In addition to the seven fatalities, 12 people were rescued, including Kevin and Peter at the Yarra Glen intersection, a driver pulled from the water at the Cotter Road crossing of Yarralumla Creek, and several people from cars stranded south of Hindmarsh Drive.

Fifteen people were injured. About 60 cars were washed off the flooded roads.

The storm dumped an estimated 95mm of rain in less than an hour over the recently-built suburbs of the Woden Valley. The water in the Yarralumla Creek drainage channel rose quickly, while the pipes under the intersection became blocked by debris. The resulting flash flood surged over the intersection and along the drainage channel next to Yarra Glen, and backed up into the Long Gully creek drainage channel along Yamba Drive. The fast-flowing floodwaters were up to 200 metres wide and 1.5 metres deep over the intersection, with the velocity of the water about 35km/hr.

The worst flooding occurred on Yarra Glen, between the Carruthers Street bridge and the Yarra Glen intersection, however the floodwaters extended from the Royal Australian Mint to Yamba Drive, several hundred metres beyond its intersection with Hindmarsh Drive. The water over Yamba Drive was over half a metre deep.

Property damage was reported all over the Woden Valley, from the Yarralumla Woolshed, where the caretaker's cottage was flooded by over a metre of water, to Torrens and Farrer. Some 500 people were directly affected, with the insurance damage estimated at $9 million.

Five police officers were awarded medals for bravery for their rescue efforts. Constable Jeff Brown was awarded the British Empire Medal for Gallantry. The Queen's Commendation for Brave Conduct was awarded to Constables Peter Harrison, Michael Lucas, Blen McInnes and Joseph Whelan.

At the inquest the coroner returned a finding of accidental drowning for the seven victims. Investigations also examined the construction of the Yarra Glen intersection.

The low-level crossing over the drainage channel and adjacent roads had been built on the flood plain of Yarralumla Creek, despite warnings from local graziers about potential flash floods. The drainage system simply could not cope with the Australia Day deluge, well in excess of a one-in-100-year flood.

A year later the intersection was replaced by the current elevated roundabout and a high-level bridge over the drainage channel, and the adjacent roads were rebuilt at a higher level.

In about 2007 Constable Blen McInnes erected seven wooden crosses near the site of the tragedy. These were replaced by a formal memorial in 2010, featuring a standing stone engraved with the names of the victims, a smaller stone with a map of the intersection as it was in 1971 and an information board outlining the details of the disaster.

It includes seven trees, one for each victim, and a seat for quiet reflection.