The father of two-year-old Nikki Francis-Coslovich, who was found dead in a Mildura house in Victoria's north-west, says child protection had been monitoring the family.

John Torney, the partner of the toddler's mother, was taken into custody on Friday morning over Nikki's death.

Torney, 31, lived three doors away from the home where Nikki lived with their mother, Peta Anne and sisters, and where Nikki's body was found in the roof cavity.

Torney, 31 is being questioned by homicide detectives, police said.

Nikki's father Nicholas Coslovich told 730 that the toddler's sisters and their mother had been living with him and his mother Helen Coslovich, until February.

He said their mother moved out with the children after he had smacked one of his daughters, causing child protection to get involved with the family.

"They were monitoring my circumstances, Peta's circumstances and all the circumstances surrounding my children, with the interests of keeping my children safe," he said.

"But they failed one of them."

Mr Coslovich said there was a child protection worker called "Angela" who raised questions about how the children were bring brought up.

He said Nikki's mother have been given "several warnings" about the condition of the family's home.

"In some of the conversations I had with Angela with Nan present, there was faecal matter right near where the kids were eating their dinner," he said.

"That wasn't one-off, that was a number of occasions where they would have a look at the condition of the place."

Nikki Francis-Coslovich's death a 'complex' case

Nikki was reported missing from her Oram Court home on Tuesday.

Local residents and emergency services helped search for the girl, but she was found dead in the home hours later, at around 6:00pm.

Police earlier described the investigation into Nikki's death as "complex" and officers are yet to reveal the cause of death.

Helen Coslovich said she could not believe what had happened.

"I cannot get my head around how [you] could do such a thing to any little person," she said.

"You don't hurt them, give them a little tap on the bum if they need it, yeah fair enough, but don't abuse them.

"You don't do that."

Nikki's family described her as a bubbly little girl who would "light up the room".

The family is now coming to terms with how to pay for her funeral, and have started web pages to help raise money.

"I can't handle this, and yet we've got to go through a funeral yet. Oh god," Helen Coslovich said.

"The more I think about it, the more it hurts, there's no way we can get that back, the amount of happiness that little girl has taken from all of us, you can't get that back."

Nikki was last seen by her mother around 11:00am on Tuesday and police were called around 3:15pm.