A 13-year-old boy has been charged with murder following the stabbing of a man in western Sydney on Friday, police said. Emergency services were called to the suburb of Mount Druinn on Friday evening after a report of a man being attacked.

Police found the 36-year-old man with stab wounds to the torso. He was treated at the scene by paramedics but died at the scene. The boy was arrested on Sunday at the Mount Druitt police station and charged with murder, breach of bail conditions and an unrelated offence of robbery.

Police said on Monday that the boy was known to police and that he had been identified by an investigator from surveillance footage at the scene. A second boy, aged 16, was arrested on Sunday night at a house in Cabramatta. He is charged as an accessory after the fact to murder.

At a press conference on Monday, a senior officer said: "In my 30-year police career, I consider this to be absolutely senseless." According to initial investigation findings, the incident arose from a "chance encounter that was completely innocent", said the officer.

The officer explained that the man was returning home from work and had stopped to speak with a group of young people he knew through family and friends. He had then gone to an ATM to withdraw cash so that the young people he knew could return home that night. A second group of young people, he added, took issue with the conversation. That group is said to have stabbed the 36-year-old man twice.

Officials have urged anyone with information, including surveillance footage or dashcam footage, to come forward, stressing that the investigation is ongoing. A detective told reporters over the weekend that detectives were investigating the alleged murder and that there were about 12 people in the area at the time of the attack.

Police said on Monday that they were aware of at least two people who were allegedly involved in the incident and urged them to surrender. The knife used in the attack was found after the incident in a drainage channel in front of a residential building in Cabramatta.

Both boys were denied bail and are due to appear in the Children's Court on Monday.