Coach Michael Bohl is delivering a stark message to Australia’s most decorated Olympian, Emma McKeon, as June’s Paris Olympic trials approach. Such is the unprecedented strength and depth of the women’s 100m freestyle in Australia that unless McKeon is at her absolute best, she may not qualify to defend her Olympic 100m freestyle title in July.

Bohl expects a time under 52 seconds will be required to secure one of two individual places in the Australian team for the 100m freestyle. McKeon won the gold medal in Tokyo in 51.96 seconds, an Olympic record and the second fastest time in history. Only two women have ever broken 52 seconds but Bohl has told his highest-profile charge: “If you don’t go under 52 seconds, there’s a risk that you won’t make it.”

The message has to be hard, because the reality is brutal – the women’s 100m freestyle is shaping up to be the toughest Olympic qualifying event for any Australian, in any sport, in history.

There are six women in the field who have won Olympic or world titles or world championship medals. Beyond McKeon, there are the Campbell sisters Cate and Bronte, Mollie O’Callaghan, Meg Harris and Shayna Jack. McKeon, the Campbells, O’Callaghan and Jack are all ranked among the top 10 100m freestylers in history, and Harris is just outside (13).

At the Australian championships on Thursday night on the Gold Coast, a prelude to the Olympic trials in eight weeks’ time, O’Callaghan won in 52.27s, just 0.19s outside her best, from Harris in a personal best of 52.59s, with McKeon third in 53.09s.

At the trials, only the top two will qualify to swim the individual event in Paris. The next four may win selection for the 4x100m freestyle relay, where the Australian women will be the overwhelming favourites to claim a fourth consecutive Olympic gold medal. And not even that is guaranteed; Australia also has the 17-year-old world junior champion Olivia Wunsch and silver medallist Milla Jansen lurking dangerously.

“The depth is crazy… there’s so much depth that it pushes us to do more and more and get better than each other,” O’Callaghan says. “Every time we race in an Australian or Queensland meet, there’s so much pressure because we have the best swimmers in the world. I’m not going to lie, it’s better than an Olympic final, the standards there. The majority of the women do 52s, so there’s always that expectation to be fast.”

Sprint coach Simon Cusack, who guided the Campbell sisters to the top and coached Jack and Harris in their formative years, also expects the Australian trials will be a higher standard than the Paris Olympic final.

“I think to safely qualify for the relay you are probably going to have to go around 52.50s [which was good enough for the individual silver medal in Tokyo] – it’s going to be tough,” he said.

Three generations of world-leading swimmers will collide in Brisbane, from Cate Campbell (31 and trying for a fifth Olympics), Bronte Campbell (29) and McKeon (29), to Jack (25), Harris (22), and O’Callaghan (20), with Wunsch (17) as a potential smokey. This extraordinary cohort is the outcome of more than two decades of work by Australian coaches, with each generation building on the shoulders of those who came before to create a dynasty of female sprinters.

Cusack, now Swimming Australia’s technical lead, said Shannon Rollason (who coached Jodie Henry and Alice Mills) and Stephan Widmer (Libby Trickett) set the standard in the early 2000s and were generous in sharing their knowledge with younger coaches through national relay and event camps.

“It’s also the fact that these athletes, in order to be part of the women’s 4×100, if you can get into that relay, there’s a fair chance of coming home with Olympic gold medals,” he said. “That’s been a major drawcard of kids preparing for these events.”

Bronte Campbell says she is exceptionally proud of the record the Australian women have established over the last 20 years.

“Because before us, there was Libby and Jodie and Alice, and then there was our generation,” she says. “Now there’s a new generation coming as well. And it really feels like we’re handing on these little legacies, from years and years and years of building up this race to be so incredibly competitive. I really enjoy being part of that. I’ve loved seeing the younger ones come through.

“I remember meeting Mollie when she was 15, and getting her involved in the squad for the first time and doing stuff on event camps together. And she was just a little tacker, and now we see her coming into her own.”

These swimmers could be daunted by the level of competition, but instead they are excited by the challenge. None of them is backing down and settling for relay spot, and they are all shooting for that individual place.

“I’ve always said that tough competition makes everyone a lot better… It’s brilliant for our relay,’’ Bronte Campbell says. “My whole life, I’ve competed against my sister in the lane next to me in training and that’s made me a lot better. So I always think that tough competition, while it’s probably more stressful than if there was no-one around, it genuinely makes you better.

“There’s no room to hide in the 100m freestyle. There’s obviously relay spots available in that event as well, but… I didn’t come back to just fill up a relay, I came back to see exactly what I could do, and hopefully that’s an individual spot.”

Former world record-holder Cate Campbell sees the irony in the fact that she will have to hold back the wave she has largely created in order to compete in the 100m at her fifth Olympics.

“I recognise that I’m going to have to perform at my best if not better than my best to a) qualify for the team, and b) stand up among the best in the world, which has always been my goal,” she said. “I inherited [Australia’s sprinting legacy] from Libby Trickett and Jodie Henry and Alice Mills and I feel like I’ve helped drive the women’s 100m freestyle globally. I’ve made it harder for myself, but I feel like there might be more to give.”