When neighbours saw Fernando Manrique pulling up tiles on the roof of his home, they had no idea a network of pipes was being installed designed to pump a poisonous gas through the family home.
Days later Manrique and his family, including two children, were found dead inside the home. It's believed they were gassed to death.
NSW Police are treating the deaths as a murder-suicide, saying the tragedy was no accident.
"There was a lot of pre-planning that went into this," a police source told The Daily Telegraph.
Police noted that none of the victims had any 'visible injuries'.
Fairfax reports police found two cylinders outside the family home, which led to them uncovering an elaborate gas network.
"It is early days in the investigation and investigators will need to wait for toxicology results and results of the post-mortems to determine cause of death," a police statement read.
The body of Maria Claudia Lutz, 43, was found in one room with her one of her children, while a second child was found alone in another part of the house.
Manrique's body was also in a separate room.
Distressed neighbours have spoken of the Colombian parents’ struggle with the children. Both 11-year-old Elisa and 10-year-old Martin were autistic.
Residents of the quiet northern Sydney suburb said Mr Manrique was a businessman, and a friendly, doting father.
They described Mrs Lutz as a protective mother, who had spoken of the hardships of raising two children with autism.
It is believed the couple’s marriage was failing.
The family home in Sir Thomas Mitchell Drive is now a crime scene.
Police were alerted about concerns for the family after Ms Lutz and her children failed to arrive at Wahroonga’s St Lucy’s Catholic School on Monday. Ms Lutz was scheduled to work in the canteen.
Ms Lutz had written about the joys and stresses of raising two autistic children on her Facebook.
"People with autism have the needs and desires as you and me but they just see life in a different way," she wrote on Autism Awareness Day in April.
"My kids have taught me that giving small steps will take me to where I want to go and I will enjoy the journey even more."
Neighbours expressed complete shock over the deaths of the "seemingly happy" family.
On Monday morning, police officers forced entry to the home and made the grim discovery. They found the family's dog had also been killed.
"It is a horrific thing that has happened in this suburb," said Supt Dave Darcy of Northern Beaches Police.
"Especially when it's kids [involved], it just breaks my heart," said neighbour Kirsty Den Brinker.
Another neighbour commented: "You wouldn’t expect this to happen around here".
"We are just all very upset and in a state of shock. I don't think I've ever seen the staff so shocked, because they worked very closely with mum (Maria Claudia Lutz)," said Warren Hopley, principal, St Lucy's School.
Neighbours told 7News that the family have been living in the street for around 10 years and that they would see them at the local playground, where they appeared seemingly happy.
Fernando worked as an executive in the city, while his wife Maria was a former lawyer.
Police have been contacting the next of kin overseas to deliver the dreadful news, while the officers who found the bodies have been offered counseling.
They have confirmed they are not searching for any suspects in relation to the deaths.
For support and information about suicide prevention call Lifeline on 131 114.