The blue-tailed monitor, blue-tailed tree monitor or Kalabeck's monitor (Varanus doreanus), is a monitor lizard of the Varanidae family found throughout the Island of New Guinea, on the island of New Britain, and in the Bismarck Archipelago. The blue-tailed monitor is also found in Australia on the tip of the Cape York Peninsula. It belongs to the indicus group of the subgenus Euprepiosaurus. Varanus doreanus has not yet been evaluated by the IUCN.