Lauren Ingrid Flanigan, 32, was found unresponsive in her cell at Brisbane Women's Correctional Centre on Friday night, police said.
Queensland's Corrective Services said officers had responded to Flanigan suffering a medical emergency.
Paramedics were called and transported the woman to the hospital.
"Support is being offered to the responding officers and their colleagues," corrective services said in a statement.
Police Minister Dan Purdie said the matter, which is "still unfolding", is under investigation by the Corrective Services Investigation Unit.
"It's awfully sad, the whole circumstance of the situation, and that matter is under investigation," he told reporters in Brisbane on Saturday.
Flanigan was being held in custody until her next court appearance in July after being charged with murder on Monday.
Neighbours had called triple-zero after seeing a young child unresponsive in the front yard of a Moore Park Beach home, north of Bundaberg, about 4.45pm.
When emergency services arrived at the home, they found a three-year-old girl, identified as Flanigan's daughter Sophia Rose, with several wounds.
She died at the scene.
A crime scene was established at the property, where police seized a knife.
Police had said two other children, aged one and two, were also at the property at the time, but neither was injured.
Both are in the care of their father.
Acting Superintendent Brad Inskip said at the time that mental health and any prior interactions with police would likely form part of the investigation.
Supt Inskip said first responders had encountered a "horrific" scene.
"It's a terrible, terrible incident for everybody," he said on Tuesday.
A GoFundMe has been established for the children and their father, with more than $48,000 raised.
Family friend Connor Thompson, who has organised the fundraiser, remembered Sophia as a sweet child with an infectious laugh that could light up the room.
"Her smile was like a ray of sunshine and her heart was as pure of gold," the fundraiser said.
"Sophia always wanted to help and was the best big sister to her baby brother and sister.
"Though her time here was short, the love she gave and the joy she shared will stay with us forever."
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