THE alleged murder of a seven-year-old boy by his mother and her boyfriend was one of the most harrowing and distressing cases senior NSW police have ever seen.

The boy was found dead inside an Oatley unit earlier this year after his mother and her partner called Triple 0, claiming he had fallen off a pogo stick and struck his head.

But as police yesterday charged his 25-year-old mother and her 29-year-old boyfriend with the child's murder after a lengthy investigation, the head of the NSW Police homicide squad Detective Superintendent Mick Willing alleged the case was among the worst he had seen.

media_camera A sign at the address where the boy was found dead.

"These matters are going to be before the court shortly so I'm limited in what I can say (but) the circumstances surrounding the death of this child involve some of the most distressing acts that we've ever seen," Supt Willing said.

Police were called to the unit on May 21 and found the child dead in his bed. A police statement of facts previously tendered in court said an autopsy showed three fractured ribs, a bruise in the shape of a fist, injuries to the boy's feet and multiple bruises to his legs, back and buttocks that were inconsistent with normal childhood injury.

Police alleged the boyfriend repeatedly abused the boy earlier this year by assaulting him, inciting others to assault him, striking him with implements, withholding food and water and making him sit outside in his underwear.

media_camera Posties stop at the crime scene where the boy's body was found.

Police will further allege the boyfriend filmed some of the alleged abuse.

The boy and his two younger siblings had been previously well cared for but police allege that changed dramatically after the family moved early this year to the man's Oatley studio, which was windowless and had no toilet facilities.

Several campers allegedly witnessed the boy being abused during the family's camping trip in March.

He was seen sitting outside his tent one morning in cold weather, sobbing and wearing just a singlet and a towel around his waist, court documents shows.

The boy approached a camper in the amenities block and gestured to be lifted up towards the tap.

Once placed under the tap, the boy "drank and drank", the camper told police.

Campers also allegedly witnessed the boy being beaten with a tree branch and forced to run laps until he fell face down on the ground. They contacted Wollongong police, who began an investigation.

But two months later the boy was dead.

The woman was refused bail, while the man is due to face court today.