He earns his crust as a tradie on Queensland's Gold Coast, but Matthew Levings used to represent Australia in the pool, and for years trained alongside a certain Sun Yang.

Key points: Matthew Levings, who represented Australia at 2014 Commonwealth Games, trained alongside Sun Yang for five years

Matthew Levings, who represented Australia at 2014 Commonwealth Games, trained alongside Sun Yang for five years Levings says China's Australian swim coach, Denis Cotterell, would never train swimmers he knew to be doping

Levings says China's Australian swim coach, Denis Cotterell, would never train swimmers he knew to be doping He says he sympathises with Mack Horton, but insists Sun is innocent until proven guilty

China's controversial swimmer Sun finds himself in a harsh limelight at the world swimming championships, getting into an altercation with British swimmer Duncan Scott over another podium snub after Australia's Mack Horton reignited their rivalry by standing behind the winner's dais after the 400m freestyle final.

But well before the controversy, Sun and a host of Chinese swimmers used to regularly train at the Miami Swimming Club on the Gold Coast under Denis Cotterell, one of Australia's most respected coaches.

That arrangement hit a snag when Swimming Australia met with Cotterell to tell him Sun was no longer welcome at Australian pools while he served his three-month doping ban back in 2014.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s Tensions were high as Mack Horton refused to stand next to Chinese swimmer Sun Yang

But before then, Sun used to train alongside a number of Australian hopefuls in the Miami pool, including Levings.

Levings, 25, represented Australia in the 1500m event at both the 2012 world short-course championships and the 2014 Commonwealth Games in Glasgow.

"I trained with Sun Yang for about five years, swam with him every day, and you'd get pretty close to these people," Levings told ABC Gold Coast.

Matthew Levings represented Australia in Glasgow at the 2014 Commonwealth Games. ( AAP: Dean Lewins )

"There was a good community of Chinese guys that would come over. Very, very hard trainers, so that's always a good thing.

"We didn't hear many stories from home from the Chinese, but they were very driven in their goals, they just wanted to be the best."

Levings has now completely moved on from his swimming life and is now a tradie on the Gold Coast.

Levings says Cotterell — who now coaches the Chinese team and has repeatedly defended Sun amid continued doping accusations — would not be coaching swimmers if he knew they were doping.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 32 seconds 32 s Sun Yang vented his anger after another podium protest, this time from a British swimmer

"Denis is a great man for one, and he is a firm believer in a clean sport. He is a very passionate guy when it comes down to that," he said.

"He wouldn't be involved in that. He sees that these guys are the hardest trainers in the world, and he sees that as why they're winning their races."

'WADA should have streamlined the process'

Levings said he sympathised with Horton's stance, but said there were two sides to Sun's story.

"There are two firm sides and I think they're both right until something comes to a head," he said.

Sun Yang's wins at the world swimming championships have been dominated by podium protests in South Korea. ( Reuters: Kim Hong-Ji )

"There's the side of Mack [Horton] where he's standing up for clean swimming and pure sport. That's a great thing, because you can be on the receiving end of something like that.

"Say if Sun Yang's trial goes ahead and Sun gets stripped of his medals, then if you're fourth, you don't get to stand on the dais, and if you're second like Mack was, you get bumped to first.

"I mean, who wants to get a gold medal in the mail?

"But then you have to follow the process. You're innocent until proven guilty, right?"

He said part of the blame for the saga must fall on doping authorities.

"I think WADA [the World Anti-Doping Agency] should have sorted this issue out earlier. This case has nearly been around for a year now. If they had streamlined that process, this would have all been sorted out prior to the meet," Levings said.

"It would have been good if it was just gold, and everyone would be happy."

'He's a passionate guy'

Levings does not keep in touch with the Chinese swimmer — he says he stays out of swimming circles nowadays — but said Sun's altercation with British swimmer Scott following the men's 800m event was part of the swimming star's character.

"That's really Sun's character, he's a passionate kind of guy — he's a blokey bloke, that's for sure," he said.

"If he sees something he doesn't like he'll say something about it.

"There's a way to go about things. Sun's from a different culture, so what we see as OK, they might not see as OK."