The size of the task facing Australia's 100m sprint freestyle swimmers on day five of the Australian Swimming Trials could not be understated.

To even reach the final was a huge obstacle facing some of the fastest women on the planet.

Eight of the women named on the start list for Friday morning's heats at the Brisbane Aquatic Centre have an Olympic gold medal safely stashed in their locker.

That includes all four of the final swimmers in the victorious 4x100m relay squad in Tokyo: Cate and Bronte Campbell, Emma McKeon and Meg Harris, plus heat swimmer Mollie O'Callaghan.

Six women — O'Callaghan, Shayna Jack, McKeon, Meg Harris and the Campbells — entered in a time that is below Swimming Australia's qualification mark of 53:61.

A further three entered a time of sub-54 seconds to set up an explosive battle for a spot in the relay team, let alone one of the two individual spots, including reigning world junior champion Olivia Wunsch, Brianna Throssell and Ariarne Titmus.

Something had to give.

However it happened, whoever missed out, it would be heartbreaking. 

That it was the doyen of Australian women's sprint freestyle swimming, Cate Campbell, that missed out — by just 0.01 seconds at that — was staggering.

"Yeah, I'm obviously really disappointed for Cate," said sister Bronte, who qualified fourth-fastest for the night's final.

"I think she's one of the most incredible athletes that we've ever had in this event. 

"I mean, she was a number-one freestyler, what — 10, 11, 12 years in a row? That's a feat that's pretty much unmatched in Australian sporting history.

"I know she was a bit ill leading into this and had a really rough last few weeks, so I'm sure she'll be disappointed because we all know that she's an incredible athlete and can swim a lot faster than that."

Harris was the fastest qualifier in 52.52 — the third-fastest time swum this year — fractionally ahead of O'Callaghan (52.57) and Jack (52.65).

"I knew that I was gonna have to do something big to get into the final," Harris said.

So intense was the competition that even Titmus missed the final, finishing 10th.

Reigning Olympic champion McKeon qualified sixth-fastest, just behind Wunsch.

Bri Throssell and 17-year-old Milla Jansen made up the rest of the high-quality competition.

"Making the top eight in Australia is really — it's like making an international final, almost," Bronte Campbell said.

"It's an incredibly tough field and I'm really proud to be part of it and proud that we've managed to have such a long legacy in this event."

A legacy that is ongoing.

Five of the top 10 fastest times in history have been swum by Australian women.

Sarah Sjöström's seven-year-old world record of 51.71 — one of just two times ever under 52 seconds — remains frustratingly out of reach, yet a real possibility for one of these swimmers if they get things right on the night.  

The secondary goal for all the finalists though, as with the 200 metres that provided such drama on night three, will be to secure a place on the 4x100m relay team.

There are few events in which Australia is so utterly dominant as the women's 4x100m relay — nine of the 10 fastest times in history have been set by Australian teams.

The three-time defending champion Aussies lost their World Championship title to the Dutch earlier this year, but were swimming with a reduced squad.