Police said another man had been arrested and was being questioned following "an incident" in Pendle Hill about 7.45am on Thursday.

Emergency services were called to Pendle Way to reports that a man had been stabbed and was unconscious.

The victim is real estate agent Vissa Esan, who was attacked after arriving at his Aussie Unity Real Estate office early on Thursday morning. Mr Esan's wife is believed to be pregnant with their first child.

Mr Esan was believed to be putting out the garbage in an alley that runs alongside his office at the time, and detectives are investigating whether the attack was random.

Workers at a nearby business are believed to have tried to revive Mr Esan, while another witness ran into a nearby supermarket saying a man had been stabbed and asking staff to call triple-0.

Police said they arrived to find a man suffering a wound to his chest.

He was taken to Westmead Hospital where he died, police said.

A worker at a shop on Pendle Way said there was a large police presence on the street following the stabbing.

She said she did not hear or see anything at the time of the stabbing, but a customer had told her that the victim worked as a real estate agent at a neighbouring business.

Police officers patrolling nearby arrested a 30-year-old man in Hallmark Street in Pendle Hill a short time later.

He was taken to Merrylands Police Station where he is being questioned by detectives.

Crime scenes have been set up in Pendle Way and on Hallmark Street.

Aussie Unity Real Estate has been contacted for comment.

Police have urged anyone with information to call Merrylands Police or Crime Stoppers on 1800 333 000, or use the Crime Stoppers online reporting page.