Portugal Colonial - Olympic champ O'Callaghan in tears after 200m freestyle win at Australian trials

The 21-year-old surged to the wall in Adelaide in 1min 54.43secs to easily qualify for next month's Singapore world championships, where she will be the red-hot favourite.

She became emotional during her poolside interview about the "rough couple of months" coming down from the high of the Olympics, where she won three gold, a silver and a bronze.

"You know, it was really stressful for me. And I think even though it's not the time I exactly wanted, it's just hard to, like, come back," she said as the tears flowed.

"I'd say this lead-up has been the hardest thing I've experienced so far.

"I don't think there's been anything as hard as this, coming here and trying to race and trying to, you know, defend my Olympic status in a way.

"But just happy that I stood up and just raced my main event."

O'Callaghan arrived in Paris with her 200m freestyle world record just broken by fellow Australian Ariarne Titmus and under enormous pressure.

She rose to the occasion, edging Titmus to win gold. She was also a key part of Australia's 4×100m freestyle and 4×200m freestyle gold-medal winning teams.

O'Callaghan and Titmus, who is on a season-long break, share the 10 fastest times in history.

"It's a really tough thing to take on. You feel once you become an Olympic champion, you're expected to perform every single meet," said Titmus, who was commentating on the race.

"And athletes, of course, we're incredible at what we do, but we're also humans first, and I think it's really important to let people in on that. I know exactly how Mollie feels."

O'Callaghan has also qualified for the 50m and 100m backstroke but is yet to decide whether she will race them in Singapore, with the 50 and 100m free, along with the relays, also on her radar.

Lani Pallister came second in 1:54.89 with all eight finalists underneath the qualifying time, once again reinforcing Australia's seemingly endless factory line of top women freestylers.

Another Paris Olympic champion, Cam McEvoy, clocked a blistering 21.30 to win the 50m freestyle ahead of Kyle Chalmers (21.68).

"Honestly, I can't really fault that at all. Just stoked to hit 21 three again," said McEvoy, who is heading to his seventh world championships.

In other races, 16-year-old Sienna Toohey backed up her breakthrough swim in the 100m breaststroke on Tuesday by winning the 50m breaststroke.

She was just outside the qualifying time, but will almost certainly suit up over 50m in Singapore given she has already booked her 100m ticket.

Harrison Turner (1:54.90) made the grade in the men's 200m butterfly, while Sam Short (7:40.95) was in ominous form to take out the men's 800m freestyle.

F.Cardoso--PC