Eucalyptus trees. Skyward view of Mountain ash trees Eucalyptus regnans, the tallest hardwood tree in the world. Specimens commonly reach 80 metres in height, and can reach up to 95 metres. Eucalyptus species (family: Myrtaceae), many of which are known as gum trees, form the predominant vegetation in Australian forests. They are used as sources of hardwood timber, tannin and kinos. Eucalyptus oil can be distilled from the leaves, though only 20 species are commercially exploited. The medicinal oil is used in inhalants, soaps, antiseptic gargles and lozenges. The oil is a source of piperitone from which thymol and menthol are manufactured.