Two sisters, both under 10 years, have been killed on Easter Sunday with police arresting a man at the home.
The little girls had been at a home on Longmuir Road in Watsonia with other family members when someone made a triple-zero call.
Officers arrived at the home just after 2.40pm on Sunday and found both girls had already died.
Firefighters and paramedics arrived moments later and performed CPR on the children.
"They treated the patients there for some time and administered CPR and other life saving measures but unfortunately were unable to save the patients and they died at the scene," an Ambulance Victoria spokesman said.
Police say a man was arrested without incident at the home and is being questioned at Heidelberg police station.
Detectives have not revealed what kind of relationship he had with the children, but confirmed he was known to the children and they are not looking for any other suspects.
Victoria Police Assistant Commissioner Andrew Crisp said it was supposed to be a long weekend filled with family get togethers, but now it's ended in tragedy.
"It's a time for families to get together and enjoy each other's company, and this will have a significant impact on friends, on family," he told reporters at the scene.
"I've heard here today that a number of our police members have been in tears."
Neighbours are also in disbelief about what's happened.
One couple, who live nearby, heard sirens and thought it was a car crash - until the sirens kept coming and they walked down to see what was going on.
"This is just terrible," said David, who didn't want to give his last name, as he arrived to see the street flooded with police and paramedics.
Both David and his wife knew an elderly lady who lives in the home and fear her grandchildren are the ones who've died.
"It's really sad if that's the case," said David's wife, who didn't want her name published.
"She's a lovely lady, but I thought she lived on her own."
Ben Cameron, who lives five doors down from the home, said he often saw the two girls playing in the area.
"I'm a bit scared, to be honest. I just don't know how I feel about it," he said.
"I've seen those kids before. I know the family. I've just seen them around. It's pretty frightening and scary."
Another neighbour said it had always been quiet area with a mix of older people and families.
"It's always been a quiet area for families," said Rachel Fish, who lives a few doors down from the house behind crime-scene tape.
"The wildest thing that's happened around here is the occasional teenage party."
More than two dozen police officers, both detectives and those in uniform, had gathered outside the home.
Other neighbours stood outside, watching in shock and close to tears.
- With Benjamin Preiss
