Two strangers armed only with a six-pack of beer have been hailed as heroes for stopping a dog that was savaging an elderly woman outside a home in Sydney's south.

Irene Koutsounadis, 90, is in a critical condition in St George Hospital having undergone surgery for injuries to her head after she was attacked by her daughter's American pit bull terrier in the front yard of the Sans Souci house about 7pm on Friday.

Justin (left) and Jacob Innes came to the rescue.

Her daughter, well-known multiculturalism and refugee advocate Vivi Germanos-Koutsounadis, 70, who lives at the Evans Street home, suffered serious injuries to her hands as she tried to pull her dog, Caesar, off her mother.

Neighbours said the dog had a "vice-like grip" on Mrs Koutsounadis and only released the woman after two strangers - Justin Innes and his son Jacob - who just happened to be walking past carrying six bottles of Victoria Bitter on their way to a Good Friday family dinner, came to the women's aid.