It was not until officers found two cylinders outside the home that they uncovered the elaborate gas network that claimed the lives of a mother, father and two children in a suspected murder suicide.

Detectives believe "air-borne gas" deliberately filtered through the sprawling Davidson home in Sydney's north likely killed Maria Claudia Lutz, 43, Fernando Manrique, 44, and their children, Elisa, 11, and Martin, 10. Both children had autism.

When officers found the bodies on Monday morning, the house was completely locked up with the odourless gas carbon monoxide believed to have been used in the tragic deaths.

A system running the gas from the bottles and through to the ceiling was later uncovered in what investigators believe was a "deliberate move" to kill the entire family, including the dog.

One of the main questions taunting those who knew the seemingly-happy parents, who juggled the complex needs of their children, was why?

Mr Manrique and Ms Lutz, who studied law in Colombia, moved to the Sir Mitchell Thomas Drive home about a decade ago. Mr Manrique, a successful executive who forged a career in business logistics after studying science in industrial engineering in his native Colombia, spent his spare time steadily improving the home.

He had made it safer for his children, who both had significant intellectual disabilities.

"The dad put gates up so the kids couldn't get out and put up a cabana and a trampoline and things for the kids,"  said resident Shaun Mayatt after placing flowers outside the home.

His partner, Moiran​ added: "The mum and dad were always in the garden and looking after the kids."

Neighbours said on Saturday they saw Mr Manrique in the front yard of his property working on what appeared to be the gazebo, which stands next to a trampoline and manicured hedges.

It is unclear exactly when the family succumbed to the air-borne gas.

Ms Lutz had spent Friday afternoon having coffee with her close friends, a tight-knit group of women who had children at St Lucy's Catholic primary school. They saw Ms Lutz as "the glue" that held them together.

It was one of these women who called police on Monday morning when Ms Lutz didn't turn up to her weekly canteen shift at the school. Martin and Elisa weren't in class either.

Teachers at the small school were told that afternoon that the mother, father and children were dead.

Staff and close friends, many carrying flowers and balloons, gathered at the Wahroonga school on Tuesday, as they remembered Ms Lutz as a vibrant and passionate woman who took on so much.

"She just took on so much and just kept going and fought the fight for everyone," teacher's aide Belinda Ings said.

"I don't know how she had room in her mind or her energy to know what was going on in any of our lives but she always did."

Elisa's teacher, Ed Hayes, said the 11-year-old was a beautiful and smart girl.

"Eli, Im not going to let you go," he told a memorial prayer service.

"I don't miss you because you haven't gone yet."

Relatives in Colombia, who were too distraught to speak when called on Tuesday, remembered Ms Lutz as a warrior.

"Always fighting for all and for all! My cute doll.. One more angel in heaven, an angel given to their children, life!!" sister Ana Lutz posted on Facebook.

"Loveee uuuuu fighting for everyone tirelessly."

Police confirmed on Tuesday they were looking at the possibility the family died from "the introduction of an air-borne gas" but were waiting for the results of toxicology tests.

Autism Awareness Australia CEO Nicole Rogerson said thousands of parents around the country shared similar experiences as the family.

"This horrible event, at least, highlights the significant difficulty many families who parent kids at the severe end of the spectrum go through," she said.

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