Australia's Bronte Campbell (left) and her sister Cate check their times after a 50-meter freestyle heat at last year's world swimming championships in Kazan, Russia. Ap File

Cate and Bronte Campbell are quite the sister act.

The Australian swimmers are both leading medal contenders heading into the Rio Olympics, though they quickly point out this isn't one of those sibling rivalries from the Serena versus Venus Williams mold, the kind that stirs mixed emotions when they inevitably face each other for the same prize.

"It's very different than Venus and Serena because we're not playing against each other on the tennis court," Bronte Campbell said on Tuesday.

"We're in a swimming pool against six other people in the race. I'm not really racing against Cate. I'm racing against myself and trying to do my best race."

Still, they are keenly aware of just how remarkable it would be to stand on a medal podium together.

"Look, it's an incredibly special thing just to be competing in an Olympics together," Cate said.

"In 2012, after we qualified for the team together in the 50 freestyle, we both knew we achieved something great just by qualifying for the team."

Bronte, 22, is the reigning world champion in the 100-meter freestyle, but 24-year-old Cate is the Olympic favorite after setting a world record of 52.06 seconds just last month.

The dazzling performance caught everyone off guard, no one more than Cate's little sister.

Bronte described herself as "a little bit astounded," before going on to say: "It wasn't something that we thought was going to happen. I certainly didn't expect it to happen. Our coach didn't. Cate didn't.

"I couldn't believe that it happened. I wasn't even at the pool when it happened. I got a phone call about it. I thought my dad was joking."