Summer McIntosh has laid down a marker for Australian rival Ariarne Titmus, qualifying for the Paris Olympics with the fastest 400 metres freestyle time in the world this year.

McIntosh, 17, finished streets ahead of her domestic rivals in Toronto, winning by nine seconds in a world-leading 3 minutes and 59.06 seconds.

That is fractionally faster than Titmus's 3:59.13 set at the Australia Open championships on the Gold Coast in April.

However, in an ominous display of confidence, the Canadian teen phenom said she was disappointed with her swim.

"Overall, I'm gunna be honest, I'm not happy with that time," she told CBS.

"I know I can go faster, [that time] doesn't really reflect my training."

McIntosh is the former world record holder in the event, breaking Titmus's mark at last year's Canadian trials with a 3:56.08.

That set up a mouth-watering three-way clash at the Fukuoka World Championships between Titmus, McIntosh and American legend Katie Ledecky, who between them had swum the 22 fastest women's 400m times in history.

However, Titmus re-established herself as the queen of the pool in Japan with an astonishing swim of 3:55.38, reclaiming the world record and obliterating the rest of the field.

Ledecky finished second, over three seconds back from Titmus, with New Zealand's Erika Fairweather third and McIntosh fourth — the same place she finished in her maiden Olympics in Tokyo as a 14-year-old.

That was Titmus's second world title in the event and confirmed her as favourite for the 2024 Olympic gold in Paris.

McIntosh has a busy program at the trials, competing in seven different events: the 100, 200 and 400 freestyle, the 100 backstroke, the 200 butterfly and the 200 and 400 individual medley (IM).