The 18-year-old Immanuel College student, who left school to make his Olympic debut, clocked 47.58 seconds after trailing down the first 50m straight to become the first Australian to win the Olympic blue riband event since Michael Wenden 48 years ago.

Kyle Chalmers celebrates his victory. Photo: ESTEBAN BIBA, EPA.

Belgium’s Pieter Timmers finished second, defending Olympic champion Nathan Adrian from the US came third, while fellow Aussie Cam McEvoy came in seventh.

Chalmers, the son of former Crows and Port player Brett, was cheered on across the state and the nation, including at a vocal Immanuel College assembly just after 11.30am local time.

Australia’s fourth 100m freestyle Olympic champion – following Wenden, Jon Hendriks (1956) and John Devitt (1960) – Chalmers produced a stunning last lap to overtake the field. He turned in seventh place.

But the South Australian entered the final in great form, setting two junior world records on his way to becoming the second fastest qualifier for the final – and then setting another in the medal race.

You bloody legend Kyle Chalmers, well done mate.

— Travis Boak (@travisboak10) August 11, 2016

You bloody legend Kyle Chalmers, well done mate.

— Travis Boak (@travisboak10) August 11, 2016

What a great moment for the Chalmers family. Dad Brett 'Smiley' must be so proud https://t.co/nkboNxBinw

— Adelaide Crows (@Adelaide_FC) August 11, 2016

Nonetheless, few predicted he would upset defending American defending Olympic champion Adrian and world No.1-ranked teammate McEvoy.

“It’s unbelievable, I’m only 18,” Chalmers told the Seven Network.

“Nathan’s 10 years older than me and already an Olympic champion so being next to him was pretty daunting for me but I knew I had to stick to my guns.”

Kyle Chalmers. What.

— Mack Horton (@_mackhorton) August 11, 2016

Wow! What an effort Kyle Chalmers #Gold #SA

— Lleyton Hewitt (@lleytonhewitt) August 11, 2016

Just watched an amazing effort by Kyle Chalmers with John Howard in his office. What a great effort! pic.twitter.com/TgfpIrA4nK

— Steven Marshall (@marshall_steven) August 11, 2016

How good was that!! Unreal Kyle Chalmers, so stoked for you and the family mate!! Doing the country and Immanuel College proud #GOLD #AUS

— Billy Stretch (@billyjstretch) August 11, 2016

Chalmers was told 12 months ago to make a choice between swimming or following his father Brett on an AFL career.

Injury prone, he chose swimming, even though he harboured regrets that this was the year he could have been drafted by an AFL club.

He was still clearly in two minds, though, often seen on pool deck with a footy in his hands.

But the attitude shifted when he surprisingly beat James Magnussen at the national titles earlier this year to win the second spot behind McEvoy in the 100m in Rio.

It shifted even further after his stunning relay swim earlier this week and again when he qualified third fastest for the 100m freestyle final.

“To be able to do that tonight proved I chose the right sport,” he said.

“My outlook on swimming has definitely changed. I’m focused on swimming day in, day out, I’m not at school anymore because I wanted to do something special tonight.”

McEvoy had arrived at Rio as the red-hot favourite after clocking 47.04 seconds at April’s Rio trials – the third fastest of all time and quickest outside the 2008-09 supersuit era.

Yet McEvoy became the lastest Australian Olympic 100m fancy to walk away empty handed.

Eamon Sullivan (2008) and James Magnussen (2012) had also gone into the last two Olympics as the overwhelming 100m gold medal fancy, only to leave broken hearted.

Moments earlier, Australia’s Madeline Groves claimed 200m butterfly silver medal.

Groves finished just 0.03 seconds behind the winner, Mireia Belmonte Garcia from Spain.

Fellow Australian Brianna Throssell finished last.

Groves, 21, became the first Australian to medal in the event since Jessicah Schipper took bronze in Beijing in 2008.

“I did awesome, I don’t know what my splits were, but I left everything in the pool,” Groves said.

“I’m really, really happy with how I went.

“It’s been a bloody awesome time for me, I could have come last in that race, it doesn’t matter I’m so happy.”

Groves led at the halfway mark before the Spaniard overtook her on lap three.

But the silver medal capped a stirring comeback by the Commonwealth Games bronze medallist in the event – the three-time national champion gave away the sport in 2011 to finish high school.

She was then struck down by a chronic shoulder and neck problem in 2014.

-with AAP