When police forced their way into the Lutz-Manrique family home, there were no signs of violence or chemical odour alerting them to the horror inside.

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.