The Australian 19-year-old is a swirl of emotion after achieving what she thought was impossible.

“I was a wreck,” O’Callaghan said after creating a 200m freestyle benchmark at the world championships in Japan on Wednesday night.

“I kind of looked (at the results board) and I was a bit like ‘oh, is that me or not?’.

“I couldn’t really explain it in the moment.

“There was tears. There was happiness. Very mixed emotions.

“I am just so proud of myself to do that. It was such an unexpected moment.”

O’Callaghan’s feat in Fukuoka came as Ariarne Titmus (200m freestyle), Sam Short (800m freestyle) and Australia’s 4x100m mixed medley team won silver medals on Wednesday night.

O’Callaghan clocked one minute 52.85 seconds, eclipsing the 1:52.98 set by Italian Federica Pellegrini in the supersuit era in 2009.

The Queenslander dared not even dream of the achievement, especially given she dislocated a kneecap at training six weeks ago.

“Obviously there’s doubts coming with that (injury) … I had a modified program leading in to this,” O’Callaghan said.

She overtook teammate Titmus (1:53.01) in the last 20 metres – the pair have now both set world marks in Fukuoka following Titmus’ 400m freestyle record on Sunday night.

In the men’s 800m freestyle final, Short pocketed silver and a coveted Australian record.

Short, who won the 400m freestyle on the opening night, touched in 7:37.76, bettering Grant Hackett’s 7:38.65 in 2005.

“Grant is one of my heroes, I have looked up to him my whole life,” Short said.

“And even though I came second, I am over the moon with the time.”

Australia’s 4x100m mixed medley team – Kaylee McKeown, Zac Stubblety-Cook, Matt Temple and Shayna Jack – took silver behind China.

After being 2.08 seconds behind entering the last freestyle leg, Jack closed the gap but the Australians finished 0.46 seconds shy of the Chinese.

Kyle Chalmers issued an ominous warning: he’s perfectly prepared for Thursday night’s 100m freestyle final after qualifying second-fastest.

The 25-year-old posted 47.52 in his semi-final and will be the oldest swimmer in the gold medal race.

“I have definitely got a bit of confidence … I have been through it quite a few times,” he said.

Teammate Sam Williamson missed the medals in the men’s 50m breaststroke by just three-hundredths of a second, finishing fourth in 26.82.

Liz Dekkers advanced to the women’s 200m butterfly, ranked fourth through the semi-finals in 2:07.11, but teammate Abbey Connor (2:10.35) failed to progress.

McKeown was second-fastest into the women’s 50m backstroke final, logging a time of 27.26 in her semi before returning to the pool for the mixed medley.

But Australian duo Thomas Neill and Brendon Smith both failed to secure a spot in the men’s 200m individual medley final, ranked 10th and 15th respectively through the semis.