Mr Duncan, who has run a swimming school at Splash Palace in Invercargill for 23 years, is revealed in documentation seen by the Otago Daily Times, as the person under investigation by the commission.
Mr Duncan was believed to be a member of the Swimming Southland board, but had left it last year.
The commission said in the documents it was considering disclosing information to the Invercargill City Council. The council owns Splash Palace.
The documents said it was investigating matters which fell within the Integrity Sport and Recreation Act 2023.
Invercargill City Council community spaces and places general manager Rex Capil said this week it had not been provided any information regarding the Sport Integrity Commission investigation, and had not directly received any complaints about coaching behaviour at the facility.
It emerged in February a complaint was made to the commission.
Swimming NZ chief executive Steve Johns said in February that both Swimming NZ and Swimming Southland were aware of a complaint and had undertaken all available actions.
The complaint was with the commission, he said. The commission does not comment on cases.
Mr Duncan was a New Zealand national representative in swimming though he never went to the Olympic Games as a competitor.
He began coaching in Christchurch and moved to Invercargill in 2003, when he started swimming school H20 Dreams. It was renamed Southland Swim School and Swim Academy in 2022.
Mr Duncan coached Southland swimmer Natalie Wiegersma at the 2012 London Olympic Games, where he was an assistant coach.
Ms Wiegersma retired shortly after returning from the Games, aged 22. The Otago Daily Times contacted Ms Wiegersma, but she declined to speak to the newspaper.
Both Swimming New Zealand and Swimming Southland had no comment to make.
Mr Duncan could not be contacted.
