The 41-year-old, who lived with her partner and two teenagers, was arrested in January and charged with murdering the 20-month-old boy near Dubbo, in northwest NSW, in March 2015.

In refusing her bail in the Supreme Court, Justice David Davies noted that reports by medical experts said the child's injuries were inconsistent with being caused accidentally.

He told the court the child had blunt force injuries to the head, abdomen and anus area and suffered a broken femur.

The boy had been placed in the care of the woman and her partner, who had been at some friends' place watching a football game on the afternoon before the death.

The woman told police she bathed the child and put him to bed.

Justice Davies said the child didn't like being bathed and engaged in a 'crocodile roll' and in the course of so doing was said to have struck his head in the bath.

A triple-zero call was made at 5.17am, when the boy was found to have vomited and was not breathing very well or not at all.

In arguing for bail, her lawyer said she had no prior convictions and the crown case for a charge of murder was not strong.

In refusing bail, the judge said he considered the crown's circumstantial case to be a strong one for murder.

The matter will be mentioned in Dubbo Local Court on April 26.