The mother of a "beautiful, happy, bubbly toddler" has tonight been charged with manslaughter over the little girl's death.

Jessica Greatorex, 22, was arrested at her Tarro home in the NSW Hunter region early this morning, almost two months after her 20-month-old daughter Hayley Rose suffered a cardiac arrest.

Paramedics were called to the house when Hayley was found unresponsive on June 19.

Hayley Rose died of a cardiac arrest in June. (Supplied)

Hayley's mother Jessica Greatorex was arrested this morning. (Nine)

Hayley was rushed to John Hunter Hospital with her mother following in an ambulance behind, but she was unable to be revived.

A post mortem revealed she suffered injuries to her head, torso and internal organs, including broken ribs and a collapsed lung.

Last week Ms Greatorex's partner Timothy Andrew Whiteley, 26, was charged with murder over Hayley's death.

Greatorex was taken into custody and is expected to face court today. (Supplied)

The pair had been in a de facto relationship and had recently moved in together.

Police now allege Ms Greatorex also played a critical role in the toddler's death.

Speaking to 9NEWS last week, Ms Greatorex choked back tears, saying her daughter "always had a smile on her face".

Greatorex's partner Timothy Andrew Whiteley was charged with murder last week. (Supplied)

"She just went down for a sleep and she just didn't wake back up," Ms Greatorex said.

At the time of Mr Whiteley's arrest, police said detectives were shaken by the case.

"When you're dealing with the murder of a 20-month-old baby it's even more so, especially with a lot of detectives having young families themselves," Inspector Wayne Walpole said.