Every Monday, after dropping her two children, Martin, 10, and Elisa, 11, at St Lucy's School on Sydney's north shore, she would volunteer at the canteen.

It was one of the many ways the 43-year-old devoted her time to the special needs school.

So when the Colombian-born mother didn't turn up for her shift in the canteen on Monday morning and her children weren't in class, alarm bells rang.

There wasn't an email to the office informing staff Ms Lutz's children, who had significant intellectual disabilities including autism, wouldn't be at school that day.

Calls to her and the family home a few suburbs away went unanswered, prompting a teacher at the 140-student school to contact police.

By 11.30am, two police officers forced their way into their home in Sir Thomas Mitchell Drive, Davidson, and found Ms Lutz, her husband Fernando Manrique, 44, and the two children dead inside.

Their bodies, along with that of the family dog, were discovered without any signs of injuries.

Police are keeping "all options" open while they investigate, including whether the family died by poisoning in a case of murder-suicide.

"I have specifically spoken with the homicide investigators and our investigators and we have come to a unanimous view that it is far too early to make such a conclusion," Northern Beaches Local Area Command Superintendent Dave Darcy told reporters on Monday afternoon.

"There is no one telling us at the moment what has happened.

"There is no one who has witnessed it."

Police on Monday were trying for hours to contact the couple's relatives in Colombia, where Ms Lutz and Mr Manrique were born and studied before moving to Australia.

The deaths have rocked not only residents of their Sydney suburb but devastated St Lucy's, which saw Ms Lutz as a "cornerstone" of their community.

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

"Two beautiful children, and their mum was a 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."

Martin, who was in year four, and Elisa, in year five, had very high complex needs that kept their mother up at all hours of the night, Mr Hopley said.

Yet Ms Lutz was at the school gates each morning, dropping off her children and volunteering her time as a much-loved member of the school's parents and friends committee.

Mr Hopley said he was not aware of any family issues, nor were the tight-knit group of mothers who shared a coffee with Ms Lutz each Friday afternoon.

"Mum was always a very happy person, always greeted the school kids in the most wonderful way.

"I don't think I have ever seen the staff so shocked, ever."

Teachers were told about the deaths at 1.30pm on Monday and the school's team of therapists and counsellors were on hand for the rest of the day.

Staff said Martin and Elisa were very active students and had fond memories of Elisa grabbing the hands of teachers and taking them on walks.

"She had a really pleasant personality, so did their son," Mr Hopley said.

"They were wonderful little personalities."

The last time staff saw Ms Lutz was on Friday with friends after school finished.

Mr Manrique was an executive director of business logistics company Drake Business Services Asia. Staff were too distraught to comment when contacted on Monday.

Meanwhile the two police officers who found the family dead were debriefed by colleagues hours later.

"It's extremely confronting," Superintendent Darcy said.

"We consistently have these sorts of concern-for-welfare cases and unfortunately in some cases this is what the police are confronted with."