South Australians are celebrating a golden performance from local swimmer Kyle Chalmers at the Rio Olympics.

The 18-year-old took out the gold medal for the men's 100 metres freestyle in his first Olympics, the first Australian to win the event since 1968.

Scores of children who train at the SA Aquatic Centre and Leisure Centre at Oaklands Park with Chalmers were ecstatic at the teenager's win.

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It was a similar story at Immanuel College in Adelaide where Chalmers is a Year 12 student.

Students gathered around to watch him race on Wednesday night (local time).

"It's pretty exciting down here," Principal Kevin Richardson said.

"I suspect he [Chalmers] will have little interest in his school work today ... his schoolmates are having a bit of an interesting day as well.

"I told them, you've had one lesson about hard work and perseverance, now go and try and learn another lesson today in your classes."

Chalmers was in seventh place at the halfway mark during the race but powered home to his personal best time of 47.58 seconds.

It was his second medal, after winning bronze earlier in the week with the men's relay team.

Assistant Principal Kate Benjamin at Saint Joseph's School in Port Lincoln, where Chalmers attended his early years of school, watched Chalmers' win from a tent at the Eyre Peninsula Field Days.

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"It was thrilling, very exciting to see Kyle swim," she said.

"Just the fact he was in the race was a huge achievement, but to see him actually win was absolutely brilliant."

She said most classes shut down at the school to watch Chalmers race.

"We've been posting lots of messages on Facebook to him and on our school site," Ms Benjamin said.

"I think worldwide, globally, people would be taking notice of Kyle now."

Chalmers' father reportedly works at Adelaide's Central Markets with one person texting 891 ABC Adelaide "that it was absolutely buzzing in there this morning".

"All the traders are stocked for him. Couldn't get any fritz anywhere for a few minutes while the race was on."

Chalmers' 'only chance' was to come home strong

Chalmers' emotional coach at the aquatic centre Peter Bishop said everyone was just in awe.

Chalmer's Adelaide coach Peter Bishop says the swimmer's strength was in the second stretch. ( ABC News )

"His ability to stay composed and trust himself and trust the work that he's done, and not be distracted by everyone's race plan ... it's absolutely amazing."

Bishop said Chalmers' strength was in the second 50-metre stretch of the race so he was not surprised the teenager was in seventh place at the turn.

"There's boys in that race who've got a lot more speed than him," he said.

"They're contenders in the 50 [metre] freestyle. The risk is he [Chalmers] tries too hard on the way down and takes away his strength.

"If he was going to get a place, it was going to be about how well he could come back home.

"That was his only chance to win."