Kauri gum is formed when resin exudes from a crack in the bark of the kauri (Agathis australis) and hardens on exposure to air. Pieces of various sizes, some weighing several pounds, collect in the axils of the branches and in the debris at the base of the tree. The colour of the gum ranges from pale yellow to reddish-brown and even black. Fossilised gum is harder, and usually paler and more translucent than that found in living forests. Buried at various depths, fossilised gum has been found on sites of long-extinct kauri forests. In the Auckland area it has appeared in strata older than the local volcanic rock. It is obtained north of 38° latitude, under lake beds, swamps, and sand dunes, as well as from higher ground. The discovery of two or three layers of gum in the gum-fields of the north indicates that a succession of kauri forests had flourished and disappeared centuries ago, each leaving its quota of gum buried at different depths.
Captain Cook in 1769 must have seen kauri gum on the beach at Mercury Bay, although he wrote in his Journal that the resinous substance he found there came from the mangrove trees. In 1819 Samuel Marsden reported pieces of gum lying on the ground near Ohaeawai, the site of intensive gum-digging 50 years later.