A Sydney father who fought a home intruder who subsequently died had previously posted online that he had "no sympathy" for thieves.

Key points: Francois Schwartz posted on a social media site Reddit two weeks ago

Francois Schwartz posted on a social media site Reddit two weeks ago Personal trainer and professional body builder Bradley Soper died after a scuffle at Mr Schwartz's property

Personal trainer and professional body builder Bradley Soper died after a scuffle at Mr Schwartz's property An autopsy is expected in the coming days

Francois Schwartz, 44, was woken up by his barking dogs before he went downstairs and challenged a stranger — who was identified as personal trainer and professional body builder Bradley Soper — inside his Harrington Park property on Sunday morning.

Two weeks ago, Mr Schwartz used his account on social media site Reddit to comment on a video showing an alleged mobile phone thief being restrained and dragged alongside a moving car.

"If he is committed to crime, he will next time bring a gun and target someone more vulnerable like an 80-year-old," Mr Schwartz's comment said.

"No sympathy. The person on the receiving end of his crime could also be poor, and worked hard at an honest job to buy that phone.

"What does theft do to that honest person?"

Confrontation came inside the house

It is understood the men had a physical altercation in the living room while Mr Schwartz's wife and young daughter were asleep upstairs.

Mr Soper collapsed and lost consciousness during the scuffle at the property.

Brad Soper died after he broke into the home in Sydney's south-west. ( Facebook )

Neighbours said they heard yelling from next door and later performed CPR on Mr Soper before paramedics took over.

He could not be revived.

Mr Schwartz was taken to Narellan Police Station where he was questioned for several hours.

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He was released without charge pending an ongoing investigation.

Behaviour was 'out of character'

Brad Soper competed in international weight lifting events around the world and was named Asia's strongest man in 2017.

He also ran a personal training business called "School of Strong", but the ABC has been told he had been struggling with money and a cocaine addiction.

The Bringelly man had recently been broken up with by his long-term girlfriend and had spent some time in hospital for kidney failure in the weeks leading up to his death.

Friends at the Smeaton Grange gym where he trained have described their shock after learning about the home invasion, saying Mr Soper's behaviour was "extremely out of character".

An autopsy is expected to reveal the cause of death in coming days, while police continue to investigate whether Mr Schwartz used "reasonable force" while trying to protect himself from the intruder.