When referencing The Treaty of Waitangi, it is acceptable to use either the Māori or English title. Note that the title is not italicised, and that the year is part of the title and therefore not enclosed in brackets.
Reference list entry
Te Tiriti o Waitangi, 1840. http://www.treatyofwaitangi.maori.nz/
Article 1 of Te Tiriti o Waitiangi (1840) outlines...
... (Te Tiriti o Waitangi, 1840, art. 1).