Former pop sensation Simpson has had a meteoric rise since taking the plunge to focus on the pool less than three years ago and will look to build on that success as he targets the Paris Olympics in 2024.

Simpson was part of the teams that won gold in the 4x100m freestyle relay and silver in the 4x100m medley relay at the Commonwealth Games in Birmingham, but it is his times in the 100m butterfly that have most impressed Hackett.

Hackett is a team captain for Australia in the Duel in the Pool event with the United States in Sydney this weekend, and a legend of the sport after winning two gold and two silver medals at the Sydney Olympics in 2000.

He also won another gold, three silver and a bronze medal at the Olympics in 2004 and 2008.

"It's crazy what he's done, to be totally honest, in terms of times and improvement, getting himself on the team, already having medals internationally," Hackett said about Simpson.

"To give people a little bit of perspective, the times Cody is swimming at the moment for the 100m butterfly would've won at the Sydney Olympics. That's how fast he's improved in two years.

"For someone to have a career like he's had in music, then come back to the sport and find themselves on the national team in two years, it is such a low, low probability.

"To see him do it has been outstanding and he's brought so much to the sport as well."

Simpson set a personal best of 51.79 seconds for the 100m butterfly at the Commonwealth Games trials in May.

Lars Frolander of Sweden won gold in the 100m butterfly at the Sydney Olympics in 52 seconds with Australia's Michael Klim and Geoff Huegill making up the podium.

Simpson swam the 100m butterfly final in Birmingham in 52.06 to finish fifth behind Josh Liendo of Canada (51.24) and fellow Australian Matt Temple (51.40).

After already making such rapid progress Simpson will now turn his focus to earning a spot on the national team for the Paris Olympics in 2024.

"It was only about 12 months ago that I was scraping into the final at the Olympic trials," Simpson said.

"A year later, to be able to go and represent Australia across the world and come away with medals as part of the men's relays is very special.

"It gives me hope and optimism for future campaigns and my improvement over the next few years."

Simpson will be joined by his partner, superstar Emma McKeon, and fellow Commonwealth Games gold medallists Kaylee McKeown, Mollie O'Callaghan and Zac Stubblety-Cook when the Duel in the Pool heads to the Sydney Olympic Park Aquatic Centre on Saturday and Sunday nights.

Australian Associated Press