Police have revealed that Maria Claudia Lutz, 43, Fernando Manrique, 44, and their two children, Elisa, 11, and Martin, 10 - who both had autism - may have died after gas was filtered through their Davidson home.

Fairfax Media understands that an elaborate system found at the property suggested the gas had been deliberately disseminated through the ceiling.

"Whilst the cause of death will be a matter for the Coroner to determine, police are looking at the possibility the family members died from the introduction of an airborne gas," police said in a statement.

"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."

When police entered the closed-up property at about 11.30am on Monday, they found the four bodies. The family dog was also deceased.

The family members showed no signs of visible injury, leading police to investigate whether they were poisoned.

It is understood police are working on the gassing theory after finding multiple gas bottles around the home late on Monday.

Neighbours have reported seeing Mr Manrique working on a gazebo in the front yard on Saturday. The alarm was not raised about the family until Monday morning by concerned staff at the children's school, St Lucy's Catholic primary school in Wahroonga, after Mrs Lutz didn't turn up to canteen duty.

Two police officers forced their way into the home and found the family dead inside.

The bodies will undergo autopsies in an attempt to determine exactly how the family died.

Relatives in Colombia were told of the devastating news hours after the tragic discovery, with Mrs Lutz's sister, Ana Lutz, remembering her as a warrior.

"No one can say anything different than you are a warrior!" she wrote on Facebook.

"Always fighting for all and for all! My cute doll.. One more angel in heaven, an angel given to their children, life!!

"Loveee uuuuu fighting for everyone tirelessly.

"Always in my life, in my heart!! I love you."

Neighbours pay tribute

Those who knew Ms Lutz described her as a devoted and happy mother who juggled the high, complex needs of her children with daily life.

She was an active member of St Lucy's, where her children were in years five and four.

A crime scene remained set up at the family home on Sir Thomas Mitchell Drive on Tuesday as locals laid flowers on the grass outside.

"They were family people," resident Moiran Mayatt said.

"The mum and dad were always in the garden and looking after the kids."

Her partner, Shaun Mayatta often saw Martin and Elisa playing on the trampoline in the back yard and said a lot of work had been done to the house to make it safe for the kids.

"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 there."

A prayer service will be held at St Lucy's school on Tuesday as parents and teachers of the tight-knit community grapple with the news.

"This is an exceptional family who have been at the school here for six years," Acting principal Warren Hopley said on Monday.

"Two beautiful children and their mum was very active member of the school.

"When all the children leave at the end of the day she would be out there at the exit waiting for her kids and knew the names of every child in the school.

"Just an exceptional woman."

Lifeline, 131 114

Beyond Blue 1300 22 4636