The abuse suffered by a three-year-old boy in central-west New South Wales included being kept inside an esky filled with ice, a court has heard.

The toddler was found unconscious and not breathing at a house in Oberon early last month and died three days later in hospital.

A 45-year-old man has appeared at Bathurst Local Court charged with manslaughter and recklessly causing grievous bodily harm.

He has been in custody since being arrested last week and today applied for bail.

Magistrate Michael Allen refused bail and said in his 30 years working in the courts, the allegations were the most abhorrent he had ever seen.

The court was told the boy had lived with his grandmother from birth and moved to Oberon on June 14.

It is alleged the abuse started then and included using extreme force to keep the child inside an esky filled with ice and water.

The court heard the man allegedly taped a ball in the boy's mouth, and kicked and hit him, including with a wooden spoon.

It is alleged officers investigating the case were initially told the boy had been knocked backwards by a rope attached to two dogs.

The court was told that staff at Westmead Children's Hospital said the boy had head and facial injuries that were not consistent with being knocked backwards.

The boy's injuries included bleeding in the eyes and bruising on the lower back.

The man is due back before court in November.