A humpback whale calf injured in Sydney Harbour should survive an 80cm gash because of it's thick layer of blubber and protective mother.

Australian authorities believe the propeller of a ferry injured the whale whilst it was swimming, causing a gash near its dorsal fin.

One of the ferries that commutes between the city and the northern beach suburb of Manly reported on Monday it had struck an object causing damage to its propeller blade and is now undergoing repairs.

The mother and calf have since left Sydney Harbour.