An elderly woman from Sydney's south-west has died after she was savagely attacked by her own pet dog on Thursday afternoon.

The 72-year-old from Wilton suffered "significant" bite wounds and lacerations to her arms, legs and body when she was attacked by the large brown cross-breed, police said.

She was taken to Liverpool Hospital, but her condition deteriorated and she died while undergoing surgery.

The woman's husband, a 74-year-old man, was also attacked and bitten while attempting to drag the dog off his wife.

He was treated at the scene for puncture wounds but is in a stable condition, police said. The couple's son was also present but was not injured.

Police said they believed the couple also had other pet dogs, and had owned the large cross-breed for about three years.

Rangers from Wollondilly Council have seized the dog and a report will be prepared for the coroner.