A keeper at Sydney's premier zoo has been critically injured by an elephant calf that crushed her against a fence post.

Taronga Zoo Chief Executive Cameron Kerr says the woman was washing the 2-year-old male Asian elephant in a barn when it pinned her on Friday.

Kerr says fellow keepers immediately came to her rescue and moved the elephant away before calling an ambulance.

He says a hospital listed her condition as critical but stable. The victim's name has not been made public at the request of her family.

Kerr says the attack happened in public view, although the public was never at risk. The cause of the attack is under investigation.