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.
Te Tiriti o Waitangi, February 6, 1840, https://waitangitribunal.govt.nz/treaty-of-waitangi/te-reo-maori-version/
Examples
Reference list entry
Te Tiriti o Waitangi, February 6, 1840, https://waitangitribunal.govt.nz/treaty-of-waitangi/te-reo-maori-version/
In-text citations
Narrative
Article 1 of Te Tiriti o Waitiangi (1840) outlines...
Parenthetical
... (Te Tiriti o Waitangi, 1840, art. 1).