Clients and training partners of Mr Soper have mourned him on social media, calling him a "gentleman" and an "awesome guy and coach" who "changed and inspired people".

His personal training business, School of Strong, is described on its website as "the product of over 12 years of experience as a strength and conditioning coach, personal trainer and athlete".

Among his listed athletic accomplishments is winning Asia's Strongest Man under 90kg in 2017.

Peter Tsikas, a personal trainer who worked with Mr Soper, told the ABC he was "pretty shocked" by the news of his friend's death.

"It doesn't make sense and that's why we're trying to put the pieces together, because we don't know what's happening — it just doesn't seem right," he said.

Crucial to the police investigation will be the cause of Mr Soper's death, and whether the level of force carried out by Mr Schwartz was reasonable under the circumstances.

"People are entitled to their home, and they're entitled to use reasonable force to protect themselves and their property," Chief Inspector Shane Woolbank said.

Mr Schwartz was interviewed for several hours on Sunday and was released from custody on Sunday night.

His neighbours were also shocked to hear about the incident.

"It's a quiet neighbourhood, it's not something we would expect. It's very random," resident Domenic Lombardo told Nine News.

Neighbours say they saw a man running barefoot down the street at the time of the incident. "The thing I noticed the most was he was barefoot - it was strange," a man told Network Ten.

Police have urged anyone with information about the incident to come forward.

with Josh Dye