A woman found dead alongside her family in a Sydney murder-suicide case was a fighter and an amazing mother, say friends who she worked with at a school for children with disabilities.

Key points: Family was found dead in their northern Sydney home on Monday

Family was found dead in their northern Sydney home on Monday Nothing in Maria Claudia Lutz's behaviour gave an indication anything was wrong, source says

Nothing in Maria Claudia Lutz's behaviour gave an indication anything was wrong, source says Teachers, students and friends reflected on Maria and family they have lost

Maria Claudia Lutz, her husband Fernando Manrique and their two children, Elisa, 11, and Martin, 10, who both had autism, were found dead in their home in the northern Sydney suburb of Davidson on Monday.

Police say they were gassed in an apparent murder-suicide.

Karen Hickmott is one of half a dozen women who knew Maria Claudia Lutz through St Lucy's primary school in Wahroonga.

"She was always caring about everyone else, it didn't matter what was happening in her life, she was always most worried about everyone else," Ms Hickmott told 7.30.

"She never let any of her troubles show, she was a fighter and she was tough."

So it was a surprise to them all when Ms Lutz didn't turn up for her regular canteen duty on Monday morning.

"Maria and I ran the canteen together, every Monday, for the last two years, and she was always here before me," Nichole Brimble told 7.30.

"When I turned up and she wasn't here, I knew something was wrong. And the kids weren't here.

"When we couldn't make contact, we made the call to police to ask them to go and check on them.

"Then we found out, the principal called me into his office about 12 o'clock to let me know what happened."

Maria's friends were worried when she and the children were not at school on Monday. ( Supplied )

'Maria was the glue for this group'

There had been nothing in Ms Lutz's behaviour that gave an indication that anything was wrong.

"We'd all been out for coffee on Friday and we all sat in the nursery ... and she was in such good spirits," Peta Rostirola remembered.

"She was Maria, she was loud and funny and she was so happy her life was going in the right direction.

"She'd had a meeting with the NDIS and she was so excited she was going to be getting all this help.

"She even felt guilty for taking that, she was just the most selfless person."

Sorry, this video has expired Friends of Maria Claudia Lutz speak of her devotion to her two children

The group of friends and the whole of the school community are struggling to comprehend what has happened.

"We are a very, very close family here at the school," Javiera Cori told 7.30.

"We do support each other very much, especially when you don't have family around, so we are the family."

"Maria was the glue for this group," Kerrie Dietz told 7.30. "She was an amazing mother."

"She had the most beautiful soul and she was the most resilient person I've known, the strongest person I've known, with the most beautiful smile always on her face," Javiera Cori added.

Karen Hickmott agrees.

"She was the heart of the school," she said.

"She's what held us all together and we all supported each other and fought like hell for our kids."

'She got up every day and she lived to make the lives of her children better'

St Lucy's is a school for children with disabilities.

On Tuesday the teachers, students and friends shared memories, artwork and photos — and reflected on Maria and the family they had lost.

Acting principal Warren Hopley described Maria as courageous and said the children had a quiet magic about them.

"There were times when getting sleep wasn't easy," Mr Hopley told 7.30.

"The children needed a lot of extra support and it wasn't easy for her but she was always very strong.

"She always took the positive view of life, that the children were alive and she was alive, that's the kind of phrase she would use, you know, the children were having a few issues, but they're alive."

Acting principal Warren Hopley at a memorial to the family in St Lucy's School. ( ABC News )

The women who knew her best back this view.

"She got up every day and she lived to make the lives of her children better," said Peta Rostirola.

"She fought for all of our children, they were all her children in her eyes," Karen Hickmott added.

"We're bereft without her."

"The children were such beautiful souls," Peta Rostirola said.

"They were both so gentle. Martin loved his animals, all he ever did was draw animals, he knew all the animals, and paint his birds.

"And Ellie, she loved to sing and she was always singing the latest song.

"No one would ever understand how hard it was, [Maria's] life, but she refused to ever be a victim and she hated anyone ever having any pity on her, and the kids had complex medical needs."

"All she ever wanted to do was do the best for them to give them the best life that they could have," Nichole Brimble said.