An eight-year-old boy has died after becoming trapped under a stack of concrete blocks at an under-construction church in Sydney's south-west.

Emergency services were called to the Leppington construction site at 1.30pm after reports a young boy had been knocked unconscious while playing on a pile of concrete slabs.

It appears some of the blocks had toppled, crushing the boy.

"Apparently there was a number of people out there with children, the children were playing as children do, they have wandered over to the materials," Inspector Mick Pearsall said.

"Unfortunately as kids do they played and didn't realise the danger in these things, and a number of these blocks has trapped one of the children."

They found the eight-year-old and immediately began CPR on him before rushing him to Liverpool Hospital.

Police and paramedics at the scene at Leppington. (9NEWS) (9NEWS)

He was pronounced dead a short time later.

Members of Sydney's Tongan community have arrived at the Liverpool Hospital to support the family.

Sitiveni Fine Teaupa, the pastor at the Tongan Adventist Church, released a statement after learning of the boy's death saying the community was in shock.

"The child was playing down the back of the five acre property when stored construction material fell on him," the statement read.

"Tragically, I've learned the child passed away as a result of injuries sustained in the accident after being taken by ambulance to hospital.

"I am devastated by this accident. Our whole community is heartbroken. There is nothing more precious to us than our children."

Work has been suspended at the site.