A young man has allegedly killed his mother, father and brother in an attack southwest of Sydney.

They were violently assaulted in their home on Juliet Cl at Rosemeadow about 1.30am on Sunday, NSW Police allege.

A third brother was also in the home at the time. The 30-year-old man survived the attack, and was able to make a desperate call to Triple-0.

Emergency services responding to the call were confronted with a “very grizzly” and “very bloody” scene, NSW Police superintendent Grant Healey said on Sunday.

They first came across the suspect’s 65-year-old mother, who was taken into the backyard so that paramedics could attempt to provide first aid.

“They did their best to get her in a position so they could try to save her. Unfortunately, she passed away at the scene,” Healey said.

First responders were met with a confronting crime scene after an alleged triple-murder in a Rosemeadow.First responders were met with a confronting crime scene after an alleged triple-murder in a Rosemeadow. Credit: 7NEWS

The suspect’s 64-year-old father was discovered next.

He was found fighting for life with “serious blunt force” head injuries, and taken to Liverpool Hospital where he later died.

The suspect’s 25-year-old brother was found dead when police searched a granny flat at the back of the property, where the victim lived.

It is unclear where the surviving brother was when emergency services arrived at the home.

He was taken to Liverpool Hospital, where he was treated and has since been released.

The suspect’s brother was the last victim to be found dead. His body was located in the granny flat where he had been living.The suspect’s brother was the last victim to be found dead. His body was located in the granny flat where he had been living. Credit: 7NEWS The suspect’s brother and mother died at the Rosemeadow property, and his father died in hospital shortly after.The suspect’s brother and mother died at the Rosemeadow property, and his father died in hospital shortly after. Credit: 7NEWS

Authorities arrested the suspect, a 32-year-old man, when he arrived at the home in a silver sedan about an hour after allegedly fleeing.

He was taken to Campbelltown Police Station, but no charges have yet been laid.

All five family members lived at the property together.

“This family isn’t very well known to police at all, and never for a domestic violence related incident,” Healey said. “It was a very tragic incident, but there was no way for us to predict this.

“This is an incredibly rare incident.”

Injuries suggest multiple weapons

“Looking at the injuries, we suspect that there might be more than one style of weapon,” Healey said.

He clarified that no firearms appear to have been used.

At this stage, police believe that both a bladed weapon and a weapon “that can create a blunt force injury” were used in the alleged attack.

Healey said that attacks that have been committed with such weapons are “always horrendous for people to confront”, and commended the first responders for their work at the scene.

A young man was arrested outside the home about an hour after the bodies were found.A young man was arrested outside the home about an hour after the bodies were found. Credit: 7NEWS A homicide probe is ongoing.A homicide probe is ongoing. Credit: 7NEWS

No weapon has been found yet, but Healey said that they are working with a “very extensive” crime scene.

“We’re systematically working through the granny flat out the back, and then we’ll go through the house, he said.

Police are also looking to work out where the suspect fled to before he returned to the home, and have asked anyone in the area who saw a silver sedan in the hour before 2.30am to contact police.

A murder investigation is ongoing, and a report will be prepared for the coroner.

Anyone with information is urged to contact Crime Stoppers.