The 18-year-old was well back at the turn but managed a sterling final 50m to win in a personal best time of 47:58 ahead of Belgium’s Peter Timmers (47:80).

But it was all Chalmers who said his country upbringing kept him relaxed throughout the race.

“I faded at the end and was worried I glided too long (into the wall),” he said.

“I’m only 18. I just knew I had to stick to my guns.

“They were a body length ahead at the turn so I knew I had to go hard in the final 50m.

“It’s all about staying calm.

“I think of myself as a country boy and country people have that.”

It is Australia's first men's 100m freestyle victory since Michael Wenden in Mexico City in 1968.

Australia’s Cam McEvoy finished seventh (48:12).