The 20-year-old touched in at 57.47s, fractionally outside her own world record, with Canada’s Kylie Masse, the Rio bronze medallist, second in 57.72s and American Smith third in 58.05s.

“My legs were definitely hurting in the last 20,” said an ecstatic McKeown.

“I’m sure it would have been pretty noticeable on the TV but I trained for that and I knew that I had a really strong back end and a really good chance to be on the podium.”

Masse turned first at 50m with McKeown third, but the Australian powerfully brought it home.

McKeown had a tough lead-up to the Games, with her father dying last year from brain cancer.

But she used his memory as inspiration to swim a sensational race and smash Smith’s world record at the Australian trials last month.

“It’s not necessarily what I’ve been through,” she said.

“Everyone has a journey of their own and it just so happens that mine’s been a really tough one.

“I wouldn’t have it any other way because I don’t think I’d be where I am today without all that happening.”

She is also targeting the 200m backstroke gold, having recently set the fourth-quickest time in history.