The little girl's body was found at a Cannon Hill house last week, seriously malnourished and displaying signs of sustained mistreatment.

Wednesday's arrest of stepmother Shannon Leigh White follows Willow's father, Mark James Dunn, being charged with murder last week.

White was not brought into Brisbane Magistrates Court when her matter was mentioned on Wednesday afternoon.

White and Dunn, both 43, are accused of murdering Willow on May 23.

Two days later, police were called to the house after the little girl had been found dead.

Dunn was charged that night and appeared in court the next day.

Detectives are continuing to investigate the health, care and treatment of Willow, who had Down syndrome.

"We have initiated investigations and inquiries in New South Wales, and in particular, we have advanced investigations that will continue and progress in South Australia," Detective Acting Inspector Chris Knight said.

South Australia's major crime squad is involved.

Members of Willow's family moved from Adelaide to Brisbane in 2017.

Police said they were faced with an extremely confronting scene at the home where Willow's body was found.

Her death is being investigated by Cheryl Vardon, who leads the Queensland Family and Child Commission, to determine whether the state's child safety system failed.

White and Dunn have both been remanded in custody to appear in court on July 20.

Neither has applied for bail.

Australian Associated Press