The 19-year-old South East Queenslander was pushed all the way to win a closely-contested race over country-mate Ariarne Titmus in 1:53.01 and Canada's Summer McIntosh's 1:53.65. 

O'Callaghan's performance eclipsed the former World Record of Federica Pellegrini, a performance done during the 'shiny suit' era at the World Aquatics Championships in Rome in 2009. Before tonight, it was the longest-standing women's WR on the books, having lasted 5110 days.

Titmus came into the 200m freestyle as the event's defending Olympic champion and showed top form three days ago in the 400m freestyle where she outswam USA's Katie Ledecky and McIntosh to set that event's all-time world best.   

McIntosh trimmed nearly 3/10ths off the junior record she set earlier this year.