Willow's body was discovered by police inside a house in the Brisbane suburb of Cannon Hill last week.
Police allege Willow, who had Down syndrome, died on the Saturday beforehand and lay dead for two days before paramedics were called.
Detectives are working with police in NSW and South Australia as part of the investigation.
Detective Inspector Chris Ahern said they were investigating all aspects of Willow's care, health and treatment in the days, weeks and years leading up to her death.
"We know that members of Willow's family relocated from Adelaide to Brisbane in 2017 and we believe that there is value in identifying potential witnesses in South Australia," he told media this morning.
"We continue to obtain information from witnesses who are assisting police to build a clearer picture of the dynamics relating to Willow Dunn's family."
He said the investigative team were not yet satisfied they had exhausted all inquiries.
Mark Dunn, father of the 4-year-old girl who died in the family's Cannon Hill home, was charged with murder earlier. Photo / Facebook
Detective Inspector Ahern said the "extremely confronting" scene police were presented with had provided "incredible motivation" in the investigation.
"It sets the tone and drives the investigators to get best and fairest outcome we possibly can because essentially Willow no longer has that voice," he said.
Willow was allegedly left to starve in a filthy bed in a bedroom. Her body had been attacked by vermin.
A post-mortem examination revealed Willow was "experiencing serious malnourishment, along with many other health concerns that supported sustained mistreatment".
