The family and church community of a husband and wife who were found dead in a south Sydney home on Tuesday morning are in "total shock".

Key points: The victims — both in their 70s — were dedicated and active members of the Jannali's Church of England

The victims — both in their 70s — were dedicated and active members of the Jannali's Church of England Their 46-year-old son, David Reid, was arrested by police and it is understood he lived with his parents

Their 46-year-old son, David Reid, was arrested by police and it is understood he lived with his parents Another family member, a brother, was informed and is in contact with NSW Police

Just after 8:00am, emergency services were called to the home of Graham and Diana Reid on Tudar Road, after their 46-year-old son David called police and threatened self-harm.

Police were confronted by "horrific scenes" after discovering the pair — both in their 70s — in the bedroom and lounge area of the home.

David Reid was arrested and is being interviewed by police.

It is understood the three had lived together at the same address.

The parents were dedicated members of the Jannali's Church of England, the church's senior minister Neil Fitzpatrick said.

"Graham and Diana have been members of our church for 50 years, long before my time here," Mr Fitzpatrick said.

He said their deaths were "a total shock" and "totally out of the blue" with no warning.

"The family is devastated … we're just overwhelmed with the tragedy of the situation," he said.

Police outside the home at Sutherland. ( ABC News: Nicole Chettle )

Earlier, Sutherland Police Area Commander Jason Box said police were called to the address by a man threatening self-harm.

"It wasn't until police were invited into the premises that they saw a male and female deceased at that location," he said.

"They [the bodies] were in two separate locations in the premises … we believe it was in the bedroom and the lounge room area — it's still being examined at this stage."

"Another family member has been informed … a brother, and he's being spoken to by police at the moment as well."

State Crime Command acting commander Stuart Smith said the investigation was in its infancy.

"Obviously homicide are being called to the scene to work with local detectives to sort out what occurred there," he said.

"It's tragic when this occurs but obviously it's something we're sorting through at this present time.

Acting commander Smith said in his 34 years as an officer "it never prepares you for what those officers would have been exposed to".

"I feel for them, I feel for the families of the individuals involved — it's always a horrific scene to deal with."