Chilling new detail has emerged about how a Sydney man allegedly spent months meticulously planning the murder of his family after hearing about the killing of two autistic boys in Perth.

Police discovered the bodies of a 46-year-old woman and two boys, aged 12 and 4, inside their Campbelltown home about 8pm on Monday night.

A 47-year-old man allegedly carried out the killings over several hours, then called triple-0 to alert authorities.

“All three persons sustained significant injuries, and it was a particularly violent crime scene,” Acting Superintendent Michael Moroney said on Tuesday.

He was arrested at the scene and charged with three counts of domestic violence murder.

The 47-year-old was the primary carer of his two sons who were autistic and non-verbal. Meanwhile, his wife worked full time.

Police claim the man meticulously planned this crime for months.

He allegedly killed his youngest son first before collecting the older boy from school and murdering him next.

He then allegedly ambushed his wife in the garage as she returned home from work.

Police allege the man told them he had read about a double murder suicide in Perth earlier this year, where a husband and wife killed their autistic teenage sons, then took their own lives

The 47-year-old’s case was briefly mentioned in Campbelltown Local Court on Tuesday, where he did not apply for bail.

“My client is pretty distressed at the moment,” the man’s lawyer Jawad Hosain told reporters outside court.

“It’s obviously a tragic set of circumstances for the victims, the community and the family of the victims and he’s pretty distraught.”

The man allegedly told detectives he is depressed after being diagnosed with cancer last year, although he claims to now be in remission.

7NEWS has spoken to friends of the man who say he started to withdraw socially and recently hadn’t been returning calls

Neighbours have been left shocked by the alleged horrific incident. Some laid flowers while grappling with how this happened in a quiet suburban street.

“I’m a mum, it’s unbelievable,” one said.

“They kept to themselves, they were very withdrawn … just a very quiet family,” another said.

“It just breaks my heart … I’m lost for words,” a resident said.

A neighbour who lived directly next door said the family was “so friendly”.

“I mean, I got to know them quite well, actually. I was just in shock, really. I gave them some toys. They were lovely. Whenever I went to see them, they were so friendly. They were really good,” the neighbour said.