Police are searching properties south-west of Sydney and in the Southern Tablelands as part of an investigation into stillbirths, allegedly within a religious sect.

Key points: Police are using hand tools to dig at properties in Picton and Bigga linked to the Twelve Tribes

Police are using hand tools to dig at properties in Picton and Bigga linked to the Twelve Tribes The religious movement is being investigated over alleged deaths of babies

The religious movement is being investigated over alleged deaths of babies The Twelve Tribes has faced controversy in the past over the disciplining of children

The properties at Picton and Bigga are linked to the Twelve Tribes, which has its roots in Tennessee in the 1970s before it spread around the world.

NSW Police confirmed in a statement the planned operation was ongoing and being undertaken by officers from the Blue Mountains local area command.

"No one has been arrested and no further information is currently available in relation to police activity," a spokeswoman said.

"Investigators are expected to provide an update at the conclusion of the operation."

It is unclear when the operation will finish.

The searches are part of inquiries by a strike force set up in September to investigate allegations of stillbirths.

Police teams dig for graves on a property in Picton. ( ABC News )

At the Picton property, officers could be seen digging with hand tools in a field.

The Twelve Tribes has previously faced criticism from former members who said they were cut off from family and friends.

Concerns have also been raised about disciplinary techniques for children within the communities, including spanking with a thin rod as a form of religious observance.

According to the group's website, eight families and several single men and women live at the 22-acre Picton property which is known as Peppercorn Creek Farm.

"The reason we are together is not to lead an alternative, self-sufficient life in the country, but to simply be the way our God intended people to be," the website explains.

The Twelve Tribes also runs a mobile cafe at festivals and events including Big Day Out, the Royal Easter Show and Woodford Folk Festival.