Neighbours are piecing together the events that took place in the days leading to the shocking moment a Melbourne mother is believed to have killed her three children before taking her own life.

The bodies of Katie Perinovic, her two daughters Claire, 7, and Anna, 5, and son Matthew, 3, were discovered inside their Tullamarine home, in the city's northern suburbs, on Thursday afternoon.

Perinovic's husband, Tomislav, 48, made the call to emergency services and was questioned by police before he was released without charge.

Less than 24 hours before the shocking killings, 42-year-old Perinovic had dropped off a bag of plums at the home of her Burgess St neighbours, Daniel and Vicky Schembri.

Mr Schembri, who has lived on the street with his wife for 48 years, said Perinovic knocked at the door at 5.30pm on Wednesday. He thanked her, and she said "Bye", he told NCA Newswire.

Just two days before that, the Schembris had heard the children playing happily in their pop-up swimming pool. "They were yelling and screaming and having fun and all that," he told The Australian.

The Perinovics were well-known to their neighbours. The children had wished Mr Schembri a happy new year. Kevin and Clare Harrison, who live opposite the family, had visited on Christmas Day to give the kids presents.

"We sat with them for a little while and Tom was on the floor playing with little Matthew with the cars that we gave him," he told The Australian. "He was helping the girls unpack their bits and pieces. We were just laughing and joking about stuff."

By Thursday afternoon, the children were dead, after an act that has sent shockwaves through the normally quiet community.

"Investigators do not believe the 48-year-old man was involved in the incident and police are not looking for anyone further in relation to the matter," police said on Thursday afternoon.

"Homicide Squad investigators have formed the preliminary view that the 42-year-old woman is responsible for all four deaths and on completion of their investigation, a report will be provided for consideration of the coroner."

Locals are now trying to work out if there are clues in what they saw and heard of the family in the hours before.

One local parent who arrived back from a country trip at around 4pm on Wednesday told the Herald Sun they saw Perinovic visiting another house on the street and speaking to a homeowner named Freddie in actions that now seem out of the ordinary.

A young mother who lives around the corner told the Herald Sun she had come back from her morning coffee with friends in on Thursday morning when she heard extremely loud shouting coming from Burgess St.

She is now convinced the agitated yelling at around 11.45am was Perinovic. About half an hour later, the mother says, she heard the sirens.

'WITHDRAWN'

Family friend Marie Groves said she last saw Perinovic when the mother-of-three visited her home in November for a party.

Groves said Perinovic seemed "unusually withdrawn and quiet".

"On November 29, I got her to come across and help celebrate my daughter's birthday with my kids," Groves told NCA NewsWire on Thursday morning.

"She was only there for a short period … her two older girls were very similar to my younger daughters, they were chatting and laughing – typical seven-year-olds, they were beautiful kids.

"On that day she did seem to be a bit withdrawn – that was not usual – she was very quiet … in retrospect maybe that was a pointer?"

Groves said she didn't know Perinovic's husband, Tomislav, very well as he was "working all the time".

Groves, who had developed a close bond with Perinovic as their children grew up together, said the last time she'd seen her neighbour was before Christmas, when she also seemed "a bit quieter than usual".

However, she added, nothing seemed out of the ordinary.

The Schembris said Perinovic moved next door about 25 years ago before he met his future wife.

"As soon as they got married she fell pregnant," said Mr Schembri, adding that Perinovic seemed "happy".

The Schembris said they used to see the "very active" kids all the time playing and riding their bikes in the street. The oldest, Claire, was getting dancing lessons and the youngest was about to start school.

Claire and Anna both went to St Christopher's Primary School in the nearby suburb of Airport West.

"Now, in heaven," Schembri said. "They were very good kids."

He the first indication he had that something was wrong on Thursday was seeing emergency services rushing to the street.

"We saw Tom sitting on the chair with his hands behind his back," he said.

"He was quiet — numb.

"And then they took him in the police car."

Schembri said it broke his heart think of the three kids wishing them a happy new year, eating lollies, just two weeks ago. "How could this happen?" he said.

LEFT JOB

Former colleagues of Perinovic, who was a practising physiotherapist, told the Herald Sun they were devastated to learn of the deaths.

It is understood she left the Glenroy Physiotherapy clinic unexpectedly about three months ago.

Acting Deputy Commissioner Robert Hill said police would work to support the families of those involved and ensure they get the help they need.

"I know there will be many people in the community struggling to understand and come to terms with this tragedy," he said.

"This has also been an incredibly difficult time for both the police and paramedics, who responded and attended the scene yesterday.

"This has been an incredibly heart-wrenching experience for all concerned."

Lee, a team manager at the West Meadows paramedics unit, also laid flowers at the scene and took a moment to reflect outside the house.

Speaking to NCA NewsWire, he said about five of his paramedics responded to Thursday's horror and were "shaken".

"The loss of three kids deeply affects everyone including our staff," he said.

"It's not a normal thing ... yes we have distressing elements to our role but incidents like this are rare."

On Friday, mourners laid flowers and cards at the property.

One child, Anabelle, left her handwritten note at the gates just moments before detectives left the scene with large, brown bags of evidence.

"To Claire, Anna, Matt and Katie. You always are very nice, loving, caring and Claire was my bff (best friend forever) always. Love from Anabelle," the note read.

The investigation continues.

Where to get help:

• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention

If it is an emergency and you feel like you or someone else is at risk, call 111.