Olympic champion sprinter Kyle Chalmers has revealed he is battling a degenerative back issue as he plots his path to another gold medal in Paris.

Meanwhile, Kaylee McKeown fell agonisingly short of breaking her own world record in the women's 200m backstroke in another spectacular performance at the Australian swimming trials.

Chalmers qualified in style in his pet 100m freestyle event, but said after the race he had to receive four cortisone injections earlier in the week to combat a long-term back issue that flared up again recently.

"'I've got bulging discs in my back and a bit of a degenerative spine. I’ve now had 10 cortisones in my back throughout my career," Chalmers said.

"I've really dealt with them since 2018-2019.

"I've been doing some landscape labouring up on the Sunny Coast and I decided I wouldn't go to work on the Friday and I'd rest for trials and kind of just spend the day laying on the couch, which my body's clearly not used to.

"It got pretty stiff, then Saturday morning I tried to do a dive and it just all spasmed and locked up."

Chalmers showed no signs of injury in an impressive 100m final swim that saw him clock a time of 47.75.

"Today's actually the best day it's felt. When I first got here, it was terrible," he said.

"I think I've put on a pretty brave face to be able to get through it but it's definitely been quite challenging getting in and out of bed and starting the day.

"I knew that all I had to do tonight was swim two laps of the swimming pool, like, it's really not that hard in the big scheme of things."

McKeown's time of 2:03.30 in the 200m backstroke final was only a fraction short of her best of 2:03.14 and confirmed her favouritism for the gold in Paris.

She finished ahead of 17-year-old Jaclyn Barclay, who also posted a time quick enough to qualify for the Olympic team.

Only 24 hours after Ariarne Titmus's world record swim in the 200m freestyle, McKeown was on WR pace for almost the entire swim but flagged ever so slightly in the final 50 metres to barely miss the mark.

McKeown had already booked her ticket to Paris with her win in the 100m backstroke and looks primed to add to the three Olympic gold medals won in Tokyo when she takes to the pool in Paris.

McKeown breezed through her 200m backstroke heats earlier on Thursday, comfortably clocking the fastest time at 2:08.83 without getting out of second gear.

Come time for the final, inspiration was in no short supply. The intrinsic drive came from McKeown's own frustration at her 100m backstroke final time, which was both just short of a world record and her lofty expectations.

But the smile had returned to her face after this one, the nerves making way for a mix of relief and pride.

"I mean, who wouldn't be (nervous)? You're getting up in front of the crowd, you're putting your heart on the line, you've done months' worth of hard training," McKeown said.

"You get up and try get yourself onto the team and you just want to do yourself proud. And with that comes nerves, unfortunately.

"Half the time I'm not even thinking about what I'm doing, like in marshalling before we're just having a good old laugh.

"I think if you can just enjoy yourself and not take yourself too seriously, that's the best recipe for a good swim."

McKeown admits added drive comes from the looming threat of American rival Regan Smith, who will compete in the US trials next week.

"To be honest with you, I'm just happy to still be sitting around that time. There's not many people in the world who are doing that at the moment — there's one that I can think of and she's about to get up and race (next week).

"So I'm nervous to see what (Smith) does. But I'm excited for the battle that we will have in Paris."

Teenager Barclay held off Han Fredericks and Iona Anderson, who both met the qualifying time in a quick final, to secure her spot in the team.

Barclay, a member of the prolific St Peters Western Swim Club, was still coming to terms with her accomplishment after the race.

"Just to be able to race these girls is incredible, and to achieve any athlete's dream to make the Olympics. I'm still a bit in shock to be honest."

Titmus closed out night four with an engrossing 800m freestyle battle with Lani Pallister which saw both record times short of their PBs but comfortably inside qualifying time.

In the women's 200m butterfly, Elizabeth Dekkers and Abbey Connor both beat the qualifying time to earn selection for Paris.

Fan favourite Alexa Leary is set to add the 100m MC freestyle to her Paralympic schedule after a dominant win in Thursday night's final, while Rowan Crothers will compete at his third Paralympics after taking out the men's 100m MC freestyle final.