Calculated planning led up to the suspected murder-suicide of a family in Sydney's north as investigators traced the gas used in the tragedy to an order placed weeks ago.

As the Colombian family of Maria Claudia Lutz, 43, prepare to arrive in Australia to take their loved ones back home, police are slowly piecing together what led to their deaths.

Floral tributes gather outside the Sir Thomas Mitchell Drive home in Davidson after a suspected murder suicide. Credit:Nick Moir

It is understood Mrs Lutz's husband Fernando Manrique, 44, had set up an account with a gas company, that allowed him to order carbon monoxide in recent weeks.

It is also understood the gas was not delivered to his home but to another address. The tanks were then kept outside his property, although the family did not know the deadly purpose for which they were to be used.