A man has pleaded guilty in a Sydney court to killing his partner's four-year-old son.

The man, 23, appeared in the Supreme Court in Sydney on Monday where he pleaded not guilty to murdering the child near Dubbo in central western NSW on April 1, 2011.

However he pleaded guilty to manslaughter and this plea was accepted by the Crown.

The boy's mother, who cannot be named for legal reasons, sobbed in court as she described how her son wet his bed that night and her partner insisted on going up to him because he was the "father figure".

She then heard four loud bangs and the man saying "stop it" in an "angry, agitated" voice, she told the court.

When she went upstairs to the bathroom, the mother said, "(the child) took a step towards me and fell to his knees".

When she changed his nappy and asked him if he wanted to wear pyjamas, the mother said the boy replied, "No mummy, it's too hot".

"He was hot to touch. He was glassy-eyed," the woman said.

The man then told her to "f*** off" and she went downstairs, she said.

"I was scared ... because of the previous physical violence to each other," she said.

A few minutes later after the man had returned downstairs, he went back up to check on the child and then called for the mother to come quickly.

The little boy was lying on his back with "four red dots" on his forehand, the woman said through her tears.

"All I could do was scream out his name," she said.

The child had suffered significant head injuries and he died later in hospital.

The court heard there was evidence the child had previously suffered injuries at the hands of the man and that the woman initially lied to police about what happened that night.

The sentence hearing continues before Justice Elizabeth Fullerton.