<p>McKeown touched in 27.13 seconds to edge 200m freestyle world champion Mollie O'Callaghan (27:19) on day one of the Australian trials in Sydney.</p><p>The six-day event acts as qualifying for not just next month's Glasgow Commonwealth Games but also the Pan Pacific Championships in Irvine, California a month later.</p><p>"<strong>It's good to come out tonight and get a 50 under my belt and be pushed along the way,</strong>" said McKeown, the most dominant female backstroker in the world.</p><p>The Olympic 100m and 200m backstroke champion will also suit up for those events in Sydney.</p><p>But she pulled out of the 200m medley, where she boosts the sixth fastest time ever, citing illness.</p><p>In her absence, Jenna Forrester (2:09.07) touched first ahead of the fast-improving Ella Ramsay.</p><p>Short was on fire, under world record pace until tiring over the final 50m.</p><p>He hit the wall in a personal best 3:40.67 -- the 10th quickest of all time and just outside German Olympic champion Lukas Martens' 3:39.96 world record set last year.</p><p>Olympic silver medallist Elijah Winnington came second, nearly four seconds behind.</p><p>"<strong>Jeez, that hurt so much,</strong>" said Short. "<strong>I've got all the confidence right now, I feel like I'm invincible so I went out there and gave it a red hot crack.</strong>"</p><p>Lani Pallister dipped under four minutes in the women's 400m freestyle, powering home in 3:59.72.</p><p>She is one of only an elite few to breach the barrier that also includes Canada's Summer McIntosh, American great Katie Ledecky and recently-retired Australian Ariarne Titmus.</p><p>Pallister has been in sensational recent form, setting lifetime bests across the 400-800-1500m board as she steps up to fill the void left by esteemed teammate Titmus.</p><p>Her training partner Forrester came second after backing up from the medley.</p><p>"<strong>I would have liked to been a little bit faster, so sorry for my profanities when I touched the wall,</strong>" said Pallister.</p><p>Sam Williamson (59.07) clocked the best time in the men's 100m breaststroke as he returns from a career-threatening injury, while Issac Cooper (24.46) won the men's 50m backstroke.</p>