Acting Detective Inspector Chris Knight told reporters that detectives were continuing their investigations, which had expanded into New South Wales and South Australia.

"Despite today's second arrest in this homicide, our detectives are continuing to progress this investigation," Inspector Knight said.

"We have initiated investigations and inquiries into NSW and in particular, we have advanced investigations that will continue and progress into South Australia."

Inspector Knight said the South Australian major crime squad was helping Queensland police on the murder investigation.

"... We continue to investigate all aspects of Willow's health, care, treatment in the days, weeks and years leading up to her death," he said.

"We know that members of Willow's family relocated from Adelaide to Brisbane in 2017, and we believe that there's value in identifying potential witnesses in South Australia."

Inspector Knight said witnesses were still helping police build a clear picture of Willow's family.

"Our investigative team are not yet satisfied that we've exhausted all inquiries, and we'll continue to investigate and evaluate all information so we can make better, informed decisions into the future," he said.

Post-mortem results revealed that at the time of her death, Willow was suffering "severe malnourishment" and other health issues that indicated "sustained mistreatment".

The ongoing investigation would look into whether possible culpability exists for "any other people" that had a responsibility to provide care, Inspector Knight said.

He would not be drawn on who else lived at the home, but said police had spoken to everybody who had lived there at the time, a factor that would be "pivotal" to the investigation.

Mr Dunn's case was heard briefly in Brisbane Magistrates Court, though he was not required to appear. He was remanded in custody to reappear on July 20.