China's Qin Haiyang broke the men's 200m breaststroke world record to complete an unprecedented clean sweep at swimming's world championships on Friday, while Australian teen Mollie O'Callaghan also made history.

Qin powered home in 2min, 05.48sec to add the 200m title to the 50m and 100m breaststroke golds he won earlier in the week, the first time any swimmer has won all three events in one championships.

Qin finished ahead of previous 200m record holder Zac Stubblety-Cook of Australia, who clocked 2:06.40, and third-place American Matt Fallon, on 2:07.74.

O'Callaghan also wrote her name in history when she became the first woman to complete a 100m-200m freestyle double.

South Africa's Olympic champion Tatjana Schoenmaker added the world title in the women's 200m breaststroke, while Hungary's Hubert Kos claimed gold in the men's 200m backstroke.

Britain won the men's 4x200m freestyle relay, finishing ahead of the United States and Australia.

O'Callaghan defended her 100m freestyle title, touching the wall in 52.16sec to beat Hong Kong's Siobhan Haughey on 52.49 and Marrit Steenbergen of the Netherlands on 52.71.

The 19-year-old O'Callaghan claimed the 200m title earlier in the week, beating teammate Ariarne Titmus in a world record time.

O'Callaghan said she "came into this week just wanting to have fun", but she will end it as a history-maker. 

"I'm not going to lie, it's such a weird feeling," she said.

"I didn't even know that no woman had done that and to be the first is just incredible.

"There are no words to explain it -- I'm just so thrilled," she added.

It was O'Callaghan's fourth gold of the competition, having also been part of Australia's title-winning women's 4x100m and 4x200m freestyle relay teams.

Both of those titles were won in world record times.

O'Callaghan said that her teammates had made the experience "so much easier".

"Going into previous meets, I was just so nervous all the time and worrying," she said.

"This is the first time that I've actually felt quite calm and just been enjoying every little bit."

Kos won the men's 200m backstroke title in 1min, 54.14sec, finishing ahead of American Ryan Murphy on 1:54.83 and Switzerland's Roman Mityukov on 1:55.34.

Kos has been training under Bob Bowman, Michael Phelps's former mentor who also coaches French sensation Leon Marchand.

Kos said his victory was down to "the Bob Bowman effect".

"That's as simple as it is," he said.

"I've been training with him for half a year now and we have a really, really good training group.

"Bob knows a thing or two about swimming," he added.

Schoenmaker won the women's 200m breaststroke title in 2min, 20.80sec, ahead of American Kate Douglass in 2:21.23 and Tes Schouten of the Netherlands in 2:21.63.

"I really just went into this race just having fun racing," said Schoenmaker, who won silver in the 100m breaststroke earlier in the week.

"We're racing some of the world's best."

Britain's men's 4x200m freestyle team won gold in a time of 6min, 59.08sec, ahead of the United States on 7:00.02 and Australia on 7:02.13.

amk/sco