A 12-year-old girl was left without food or water to die in the backyard of her foster home, delirious with pain and unable to move, a court heard Monday.

Crown Prosecutor Richard Coates said doctors at Royal Darwin Hospital (RDH) uncovered a "massive 1.5 litres of puss" in the child's leg, caused by a fracture to her left thigh.

Sisters Toni Melville, 43, and Denise Reynolds, 42, have been charged with the manslaughter of the girl, who first started to limp following a school sports day.

Three weeks later, on July 12 last year, she died from a blood and bone infection.

Coates said osteomyelitis was a condition common in children that doctors would have had "no difficulty diagnosing."

"With medical treatment she could have made a full recovery," he said on the first day of a Northern Territory Supreme Court trial in Darwin.

"Even if medical treatment had been sought on the last day ... when her heart stopped beating, she would have still had a fighting chance of survival."

Coates said Reynolds was a "stubborn" woman who insisted the child had sustained a sports injury which would improve with exercise.

When the girl refused, Reynolds would "smack her leg with a stick".

"(The child) was unable to stand unassisted and when she was forced to she would just fall to the ground," Coates said during opening submissions.

"When assistance was not forthcoming to help her to the toilet she would urinate and defecate in her clothes where she lay."

Coates said social workers who visited the three-bedroom Palmerston home - which housed 17 people - the day before the death found the child lying on the kitchen floor crying.

When she was told to have a shower she struggled to walk and had to use the walls for support.

Coates said the jury would also hear evidence that, hours before her death, the girl was "punished" for soiling her clothes and taken out to the backyard.

"The children were told they were not allowed to help her get food or drink," he said.

"You will hear evidence from children who were out playing in the yard that later in the day (the girl) began to talk about fairies and witches and she said a limousine was coming to pick her up ... (The girl) said yeh, I can see the light now, and she just stopped breathing."

Melville told police she called Triple 0 but Coates said records showed the first call she made was to her sister.

The child was pronounced dead at RDH later that evening.

Coates said the jury might find both women were "genuinely distressed" by the death, but the standard of care they offered her had "grievously fallen short" of what would be expected of a reasonable person.

The trial before Justice Trevor Riley continues.

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