Image from Wikipedia.
Just pipping the post a day early -- I spotted this NZ Herald article on the history of the day. Quite a good summary, I thought.
They're quite right about full-on and formalised celebrations being of fairly recent vintage -- 1934. At the 25-year mark (1865), the central North Island was still on a war-footing between Imperial and Colonial forces and the Maori iwi, so anything to do with the Treaty of Waitangi was probably a touchy subject. At some place up North called Waitangi (possibly the same place, but it's not certain), each year on New Years the local Total Abstinence Society held their completely non-alcoholic party.
By January 1880, nearly 40 years after the treaty, facsimile copies of the 1835 Declaration of Independence, Captain Hobson's draft, and the treaty as signed by iwi in both main islands, compiled by H. Hanson Turton, were published by the Government Printing Office. (West Coast Times, 23 January 1880)
In 1890, regattas, jubilee celebrations, and even Maori war dances were features of a long series of events to mark the 50th anniversary of the treaty -- just not exactly 6 February. It was more end of January, tying in with the Auckland Provincial holiday.
More on the day here at NZ History Online.
Jayne in the comments brought up a very good point about the document itself -- the Treaty has had a hard time of it over the years. From the Archives New Zealand website:
"In 1841, only a year after the Treaty of Waitangi was drawn up and signed, the documents were saved from a fire at the government offices in Official Bay, Auckland. Poor storage between 1877 and 1908 led to the Treaty being damaged by both water and rodents. However, facsimiles of the Treaty had been created in 1877, before any damage occurred and all signatures have survived. After a series of different conservation treatments, and different homes, the Treaty was finally brought to National Archives in 1989, where the documents are now on permanent display in the secure, stable environment of the Constitution Room, Archives New Zealand."The above link shows images of the treaty documents as they are today.