A man has been charged with three counts of murder after his mother, father and younger brother were allegedly bludgeoned at a home in south-west Sydney.

Police were confronted by a “very grisly crime scene” after being called to a family home in a quiet suburban cul-de-sac in Rosemeadow about 1.30am on Sunday.

The accused’s mother, 65, was dead, while his father, 64, was found alive with horrific head injuries. He died later in hospital.

The body of another man, aged 25, was found in a granny flat at the back of the property.

A 30-year-old man who survived, believed to be another brother, was found injured. He was taken to the hospital and was later released.

Police are treating the deaths as an alleged domestic violence incident. Photo: AAP

The suspect, believed to be the son of the middle-aged couple and the sibling of the other two victims, was arrested after returning to the scene about an hour later in a silver Subaru sedan.

Police arrested the 32-year-old and took him to Campbelltown police station for questioning.

On Sunday night, the man was charged with three counts of murder and one count of attempted murder over the domestic violence incident.

He was refused bail and will appear before Campbelltown Local Court on Monday.

Emergency services on Juliet Close in Rosemeadow early on Sunday. Photo: Seven Network

Superintendent Grant Healey said police suspected more than one type of weapon might have been involved, potentially including a hammer.

“We think that there’s a bladed weapon involved, as well as some type of weapon that can create a blunt-force injury,” he said.

Healey recounted the “very bloody” scene that police found when they arrived at the home.

“Blunt-force trauma is always horrendous for people to confront and any edge weapon attack is always horrendous to confront,” he said.

Police found the mother’s body first.

“They started doing first aid with her and she was taken into the rear of the yard so ambulance and police could do first aid,” Healey said.

“Police then went inside the house and found a 64-year-old male who was still alive with serious blunt force injuries, and a little bit later when police went through the premises, they found a 25-year-old male deceased in a granny flat at the rear of the premises.

“It appears that they’re all family members. It appears that it’s the parents, which are the two people in their 60s, and the brother, who was 25.

“The alleged offender appears to have fled the scene and then returned.

Healey said the weapons were yet to be recovered.

“Whilst we can say it’s a hammer, we’ve got to be 100 per cent sure, so we’ll take our time and investigate,” he said.

The alleged killer returned to the scene in a silver Subaru. Photo: AAP

Numerous police, including forensic investigators, could still be seen coming and going from the property late on Sunday morning.

Healey said the family was not well known to police and there had been no reports of domestic violence-related incidents.

“We know it’s a family unit and we’re trying to work our way through what happened in the evening,” he said.

“This is a very tragic incident … there was no way for us to predict this type of thing was going to occur.”

A report will be prepared for the coroner.

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-with AAP