Richelle Hill continues to showcase her massive triathlon potential, posting the best Australian finish at the Ironman 70.3 WA Asia-Pacific championships.

In her debut over the 1.9km swim, 90km cycle and 21.1km format, Hill finished fourth on Sunday at coastal Busselton.

It continues the Hill family's impressive triathlon history. Her mum Rina was a star in the sport, racing for Australia at the 2004 Olympics.

Richelle's long-term goal is to emulate that achievement at the 2032 Brisbane Games.

Richelle's dad Allister also had an excellent 70.3 race on Sunday, finishing second in the 55-59 age group.

Italian Gregory Barnaby and Spaniard Marta Sanchez convincingly won the overall 70.3 titles as Busselton celebrated 20 years of Ironman triathlon racing.

Usually, the full Ironman event is the focus at the WA coastal town. But this year the 70.3, or half-Ironman, race was the main attraction because of its regional championship status.

After five years of only racing triathlons occasionally, Richelle put a much bigger emphasis on the sport this year and has racked up a string of impressive results.

Rina coaches her. Mum and daughter have a similar skill set, with an ability to contend over long-course events such as the Ironman 70.3 races as well as the shorter Olympic-distance format.

"It's a great starting point, something I can really work off," Hill said of Sunday's win.

"At this point, I do have a goal of the Brisbane Olympics ... but I do just love the long-distance.

"She (Rina) has a lot of great advice and I feel like she best prepares me.

"A lot of people think it would be challenging (having her mum as coach). Sometimes it's a little bit awkward ... but I feel we make it work. I've got the best support."

Less than two months after finishing sixth at the Hawaiian Ironman world championships, triathlon's most famous race, Barnaby pulled clear in the run on Sunday to claim his first Ironman or Ironman 70.3 win.

He clocked three hours 37 minutes 35 seconds, passing South African Jamie Riddle inside the last six kilometres of the run and winning by two minutes.

American Marc Dubrick posted 3:39:51 for third. Olympian Jake Birtwhistle was the first Australian in the men's race, finishing fifth.

While Barnaby collapsed at the finish, Sanchez still had the energy to run over to nearby family and friends for hugs after crossing the line.

The Spaniard dominated the women's race to win in 4:06:38, just over a minute up on Sweden's Anna Bergsten.

Australian-based Dutch triathlete Lotte Wilms was third in 4:08:03, and Hill clocked 4:09:10.

Barnaby's win means he takes the lead in the Ironman pro series rankings ahead of next month's last race, the 70.3 world championships in Taupo, New Zealand.

"It's amazing ... I'm at the top of my career now," Barnaby said.

"I needed a good result here for the pro series, it takes a bit of pressure off."

The Ironman and Ironman 70.3 races on Sunday attracted a combined field of about 3400 competitors.

The full Ironman race - a 3.8km swim, 180km cycle and 42.2km run - was continuing into the evening.