His royal performance of I Still Call Australia Home was linked to the 2018 Commonwealth Games on the Gold Coast, his home city,

"Because I am from the Gold Coast, they had me go over and sing I Still Call Australia Home at Buckingham Palace for the Gold Coast Games," Simpson said.

"If you had told me four years later I would be swimming at the next one ... it's funny. You have just got to take it all in your stride, I guess."

Simpson will race at this year's Commonwealth Games in Birmingham after making his first Australian swim team.

The very thought astounds the 25-year-old.

Simpson's mother Angie and father Brad both swam for Australia, at the 1987 Pan-Pacific Games and 1994 Commonwealth Games respectively.

A promising junior swimmer, Simpson won two gold medals at the Queensland state championships in 2009.

The same year, he posted some songs on YouTube and was discovered by an American music manager.

His family moved to Los Angeles in 2010 and Simpson became a global pop star who also appeared on Broadway and on a host of American television shows.

In 2019, Simpson returned to the pool with an aim of perhaps making the 2024 Olympics. Instead, he's a Dolphin now.

"If you told me two years ago I would be here (on the Australian swim team) this soon, I wouldn't believe you," he said.

"I would probably punch you in the face and tell you that you're dreaming."

Simpson said his selection for the Commonwealth Games, where he'll swim the 100 metres butterfly, was "a testament to the last two years" of intense training.

"It has been up and down and I have had doubts," he said.

"I have had good weeks and really shit weeks sitting in my room wondering if I was in over my head or dreaming too big."

Throughout, he converted his pop star skill-set to swimming.

"It's all similar in a sense," he said.

"It's all having eyes and ears on you and having expectation on you and certain pressures to deliver a certain result.

"It's all like a similar kind of stimulus for your brain. And you get used to being able to get up and do it and be able to remain calm and steadfast under certain conditions.

"And that is what I had learnt to do through my whole childhood, growing up and performing in front of people and singing in front of the queen and doing all these things."

Australian Associated Press