NSW police have confirmed the Sydney family of four died as a result of murder-suicide, likely from air-borne gas.

The bodies of Colombian family Fernando Manrique, 44, Maria Claudia Lutz, 43, and their two children Elisa, 11, and Martin, 10, were found on Monday morning after police forced entry into their Davidson home.

St Lucy's primary school, where both the children attended and Ms Lutz helped out, raised the alarm on Monday when the kids failed to turn up to class and Ms Lutz didn't arrive to run the canteen. 

Both children were deaf and lived with non-verbal autism.

The exact cause of death has not yet been determined, but police confirmed that the family likely died from the introduction of an air-borne gas delivered via pipes rigged throughout the roof, in what they described as an "extensive, elaborate and well-planned" operation. 

"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 air-borne gas," they said in a statement.

News Corp reports that neighbour Ofik Thomassian, 72, who lives directly across the street, saw Mr Manrique working on the roof on Saturday.

"He was right up the roof replacing tiles and using power tools of sorts," she said. "He was cutting and banging and making all sorts of noise and pulling up tiles but I don't really know what he was doing."

The family's dog was also found dead. 

Photos: Facebook.
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