Mollie O'Callaghan became the first woman to win 100m and 200m freestyle titles at a World Championships ©Getty Images

Mollie O'Callaghan has become the first woman to win both the 100 and 200 metres freestyle titles at a World Aquatics Championships as she prevailed in the shorter discipline today in Fukuoka.

The Australian 19-year-old triumphed with a time of 52.16sec to beat Hong Kong's Siobhan Bernadette Haughey and Dutchwoman Marrit Steenbergen by 0.33 and 0.55, respectively.

"It's so weird, I am not going to lie, it's such a weird feeling," O'Callaghan said after defending her 100m title from Budapest last year.

"I didn't even know that no woman has done that. 

"And to be the first, it's just incredible.

"There's no words to explain it, I am just so thrilled.

"I am just trying to keep my emotions intact and keep it controlled at the moment and taking it day by day and I haven't really thought too much about everything."

Mollie O'Callaghan defended her women's 100m freestyle title from last year with a time of 52.16 ©Getty Images
Mollie O'Callaghan defended her women's 100m freestyle title from last year with a time of 52.16 ©Getty Images

Another record fell in Fukuoka as China's Qin Haiyang set a new world best time in the men's 200m breaststroke final as he clocked in after 2min 05.48sec.

The performance cut five-tenths off the previous best while Zac Stubblety-Cook of Australia did enough for silver in 2:06.40 and the United States' Matt Fallon took bronze 2.26sec off the pace.

He was still not completely satisfied with the achievement however.

"It's not my dream," Qin said. 

"I dream of a 100m world record."

The last men's gold medal of the day went to Hungarian Hubert Kos in the 200m backstroke as his 1:54.14 time ensured that he got the better of American Ryan Murphy and Roman Mityukov of Switzerland by 0.69 and 1.20, respectively.

Olympic champion Tatjana Schoenmaker added another title to her belt with a 2:20.80 win in the women's 200m breaststroke.

America Kare Douglass, in 2:21.23, and Tes Schouten of The Netherlands, a further 0.40 back, then made up the top three.

Britain's Tom Dean, James Guy, Matthew Richards, and Duncan Scott combined for a winning time of 6:59.08 in the men's 4x200m freestyle relay to pip the US and Australia to gold.

The Netherlands won women's water polo gold after a penalty shoot-out against Spain.

The two sides were even at 12-12 at the end of regulation time but a 5-4 result in the decider ensured Dutch gold.

Earlier in the day, Italy downed Australia 16-14 for the bronze medal.