Four people, including two children, were found dead inside a home on Sydney's Northern Beaches on Monday morning.
Police went to the home in Sir Thomas Mitchell Drive at Davidson at 11.20am after they had been contacted by a friend concerned about the family's welfare.
Officers forced their way into the home and were confronted by the horrific scene - the bodies of two adults, named Monday afternoon as Fernando Manrique, 44, his wife Maria Claudia Lutz, 43, and the couple's two children Martin, 10, and Eliza, 11.
Police are treating the tragedy as a murder-suicide.
According to the Daily Telegraph, those close to the family believe the demands of motherhood with two intellectually disabled children may have become too much for Ms Lutz. The young siblings' severe autism meant they were unable to speak.
Neighbours were reported yesterday as saying that Ms Lutz - a dedicated volunteer and fundraiser - and her partner waged a parenting battle that left her with a heavy heart and contemplating how to cope.
"Nothing like this ever happens around here. It just goes to show you everyone has problems and you never know what happens behind closed doors," a tearful Mick Kastrounis, who lives opposite the family's home, told the Telegraph.
"Maria used to tell me how hard it was on her and her husband having two kids who were deaf and dumb," said neighbour Sonja Perry.
"She said it was a heavy thing on her heart and some days she found it impossible to cope. They were such a lovely, friendly couple."
Ms Lutz would volunteer at the canteen at St Lucy's School on Sydney's north shore every Monday after dropping the kids off.
When the mother-of-two didn't turn up for her shift on Monday morning and her children weren't in class, alarm bells rang.
When calls to the family home went unanswered, a teacher contacted police, Fairfax reports.
Two police officers reportedly forced their way into the family home. The four bodies, along with that of the family dog, were discovered inside without any signs of injuries.
It is understood police are investigating whether the family died by poisoning in a case of possible murder-suicide.
Homicide detectives had joined the investigation until the circumstances were confirmed, but Supt Darcy said they agreed with local officers it was too early to say what went on in the house.
"There's no one telling us what happened. No one who has witnessed this."
He added: "I'm not willing to discuss cause of death."
Supt Darcy refused to say how long the bodies could have been in the house before they were discovered but said that the two officers who arrived first at the scene had found what they saw "extremely confronting".
The family is believed to be Colombian and moved to the area in 2005. They weren't known to police.
Mr Manrique worked for a logistics company, where he had been the chief technology officer for four years.
The bodies have since been removed from the house. The coroner was seen carrying each of the family members from their North Sydney home after 10pm last night.
"This is very serious, two young children, two adults have died ... I just want to keep this investigation nice and crisp and untainted so investigators ... can come to the truth about what has happened."
The two children were pupils at St Lucy's, a Catholic primary school in Wahroonga for students with disabilities.
Principal Warren Hopley described Ms Lutz an a "happy woman" he spoke with most days. He said the community was in shock.
"We're absolutely devastated by the news. This is an exceptional family who have been at the school here for six years," he told media outside the school.
"Mum was a very active member of the school. Almost the cornerstone of the school in many ways. She would always attend meetings, ran the tuckshop here. She was a very busy woman and (the children) had very high, complex needs."
Police said a "large" family dog was also killed.
"We're gradually working our way through these very tragic circumstances to get to the bottom of it, and find out the truth to what happened here today," Supt Darcy said.
The scene was so traumatic it left experienced officers shaken, Supt Darcy said.
"It's an horrific thing that has happened in this suburb."
Police are now trying to locate relatives of the family.
Locals not far from the street, which has been blocked off by police, have expressed their shock that their own children may know the boy and girl found dead inside.
Local mother Kirsty Den Brinke said she couldn't believe something like this could happen on her quiet suburban street.
"It is really quiet around here and very family-friendly. This is going to shake the whole community," she told The Manly Daily.
Mrs Den Brinke said she had returned from picking up a sick child from school to see part of the street cordoned off with police tape.
"I saw the road was blocked off," she said.
"It is quite shocking. It is awfully shocking. I have got kids and this involves kids. You just want to know if it is anyone that your kids know.
"My heart breaks. We are a community here and you would like to think that if somebody is in trouble they would reach out."
Where to get help:
• In an emergency: call 111
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633, or text 234 (available 24/7) or email@example.com or live chat (between 7pm and 11pm) http://livechat.youthline.co.nz/mibew/chat?locale=en&style=youthline
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155 (weekdays 11am to 5pm)
• Samaritans 0800 726 666