McKeown’s triumph is Australia’s third gold of the Games, following the women’s 4x100m freestyle relay and Ariarne Titmus in the 400m freestyle.

McKeown entered Tuesday’s final as world record holder but third-fastest qualifier behind American Regan Smith and Canada’s Kylie Masse.

And the Australian trailed her feted rivals at the halfway turn before a stunning surge in the final 30 metres delivered victory.

McKeown triumphed in 57.47 seconds, with Masse (57.72) taking silver and Smith (58.05) the bronze.

Australian veteran Emily Seebohm, at her fourth Olympics, finished fifth in 58.45.

McKeown will again meet Smith over 200m, with the Australian dropping the 200m individual medley from her program to focus on backstroke.

Her triumph comes against a backdrop of family tragedy, with her father losing his battle against brain cancer last August.

The 20-year-old swimmer dedicated her world record at Australia’s selection trials to her father.

“I use it every day that I wake up,” McKeown said at the June trials.

“I know it’s a privilege to be on this earth and walk and talk.”