A family of four, including two children with autism, who were found dead in their northern Sydney home died as a result of a murder-suicide, police say.

Police said it appeared the family died of gas poisoning and are asking the public for help to discover the motive behind the deaths.

Fernando Manrique with his daughter Elisa. ( Facebook: Fernando Manrique )

The bodies of Colombian nationals Fernando Manrique, 44, Maria Claudia Lutz, 43, and their two children, Elisa, 11, and Martin, 10, were found yesterday morning when police forced entry to the family's Davidson home.

Ms Lutz's friends, who she worked with at a school for children with disabilities, have paid tribute to her as a fighter and an amazing mother.

A NSW Police statement said they had no visible injuries and they were awaiting the results of toxicology reports to determine the cause of death.

"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," the statement said.

"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.

"Forensic police continue to recover evidence from the home in Davidson."

A large dog was also found dead at the house.

Detectives set up taskforce, search backyard, house

A crime scene is still set up at the home and detectives are back today, searching the backyard and the house.

The homicide squad was looking into the matter, but have handed investigations back to the Northern Beaches Local Area Command.

"Strike Force Josselyn has been established with the assistance of State Crime Command's Homicide Squad to investigate the deaths," a statement said.

Northern Beaches Police Superintendent Dave Darcy initially said they were keeping an open mind as to whether they were searching for a suspect, but police have since ruled out this possibility.

Colombian migrant Alejandro Osorio did not know the family but says he felt compelled to say a prayer for them. ( ABC News: Jessica Kidd )

Superintendent Darcy said the family were valued members of the community.

A crime scene has been set up in the garage of the Davidson house. ( ABC News: Jessica Kidd )

"The mother in particular is held in very high regard in the community," he said.

"Since coming to Australia they've made a significant contribution to how we live, they've done a fantastic job."

Neighbours said the children were lovely and kept to themselves.

Flowers have been laid outside the home, as neighbours and friends pay their respects.

Neighbour Janine Akers urged anyone who needed help to ask for it.

"If you're feeling like that just get help, we're all here especially in a community like this," she said.

"If someone is struggling, get help."

'Very positive, strong woman, respect by everybody'

The children went to St Lucy's primary school in Wahroonga.

Acting principal Warren Hopley said Ms Lutz needed to be remembered as a "dynamic person".

"Very positive, strong woman, respected by everybody and loved by everybody. Parents and students here were just inspired by her," Mr Hopley said.

"Having a child with disability is not always easy, there are challenges, but Maria would support people, assist them, help them to be strong.

"Show them all the positive aspects there are to having a child with a disability, whatever it might be."

He said he would have been extremely surprised if Ms Lutz had been overwhelmed by the situation.

"Sure she struggled like many parents do, particularly [with] children with disabilities, there are extra challenges.

"There were nights where she struggled to get a decent amount of sleep, she often talked about that.

Detectives continued to search the Davidson property today. ( ABC News: Jessica Kidd )

"But even so, the kind of involvement she had in the school, swimming lessons she would join in on as a volunteer, going to the tuck shop and working in there, helping with the read.

"She was just doing some extra study recently to be a teacher's aide, she is a lawyer, but she wanted to be involved in the school and get a certificate to be able to do that.

"So this is a person who was full of life."

Mr Hopley said the event was an "enormous shock".

"I haven't seen a staff just so decimated, today was an absolute struggle."

He said he spoke to Ms Lutz every day of the week, but he could not have predicted what had occurred.