Urtica frerox, Ongaonga, or NZ tree nettle is endemic, and is found throughout the North and South Islands reaching Otago as its southern limit. It is commonly found up to a height of 600 metres above sea level, in the fringes of bushland, in coastal and lowland forest margins and shrublands, where It often forms thickets.  Ongaonga is a small tree, soft-wooded, or shrub, growing up to three metres high, with a trunk about 12cm diameter. It has many branches, with the branches tending to be intertwined. The leaves, branches and branchlets all have stout stinging hairs which are silicified - the head breaks off on contact, and the poison is injected into the puncture. There are male and female flowers with the sexes appearing on separate trees. Like other parts of the tree, the flower spikes bear stinging hairs. Flowering is from late spring to early autumn.

Tree Nettle is dangerous to livestock and man. The brittle-pointed stinging hairs cause intense pain and both man and animals may even die. Horses and dogs are the only known affected animals, and horses have been known to die quite soon after blundering into tree nettles. (Some losses in both horses and dogs still occur).

Fatal poisoning in man was first recorded in 1961 when a young man died from the effect of tree nettle stinging. The facts are important enough to warrant a detailed account. Two lightly-clad men, 18 and 21 years old, went shooting in hill country of the Ruahine Ranges in the late afternoon of 26 December. On their return in the early evening they hurried through what one described as "a lot of stinging nettle and it felt like a series of needle pricks". Not long after, perhaps less than an hour, one of them complained of stomach ache and appeared to be exhausted. Partial paralysis set in when he lay down to rest; he had difficulty in breathing and a short time later he could not see. He was rescued, but died five hours later after being admitted to hospital. His companion suffered similarly, though not to the same extent, and did not die. Three Home Guard horses died from stings in the Hutt Valley in 1944.

This plant is also important because of its ecological value , as the Red Admiral caterpillars require stinging nettles to breed. Red Admiral caterpillars will eat most native and non-native nettles, but the eggs are laid almost exclusively (with the exception of a Scrub Nettle or two) on native Nettles. Yellow admirals prefer the non-native kinds including Common Stinging Nettle (Urtica dioica) and Annual or Dwarf Nettle (Urtica urens).