High Court judge Sir Edward Taihakurei Durie says the signing of the United Nation Declaration on the Rights of Indigenous People was the most significant day for Maori rights since the signing of the Treaty of Waitangi.
Maori Affairs Minister Pita Sharples this week flew to New York without publicly revealing he was to make a speech announcing New Zealand would endorse the declaration.
The previous Labour government had refused to sign, saying it was incompatible with New Zealand's constitution, legal framework and the Treaty of Waitangi.
National ministers described it as an aspirational document, which was not legally binding.
But Sir Edward, a former chief judge of the Maori Land Court, said in an email to the Maori Party if the party achieved nothing else it would have a "treasured place" in Maori history.
Maori Party MP Hone Harawira read out the email in Parliament yesterday.
Sir Edward went on to say that while the declaration had only "moral force" the same could be said of the treaty.
"Important statements of principle established through international negotiation and acclamation filter into the law in time through both governments and the courts."
Waitangi Tribunal reports, court cases, changes in legislation and official policy had been important for Maori rights, Sir Edward said.
"I would still rank the day that New Zealand gave support to the declaration as the most significant day in advancing Maori rights since February 6, 1840."
The areas of the declaration that said indigenous peoples should be dealt with through their own institutions had "potential implications" for the Office of Treaty Settlements, Crown Forest Rental Trust, Waitangi Tribunal and those developing social service policy, he said.
Mr Harawira said a draft of the declaration had been supported by "national Maori icons" including Sir James Henare, Dame Whina Cooper, Dame Mira Szaszy, the late Maori Queen Te Ataairangi Kaahu, Sir Robert Mahuta, Whakahuihui Vercoe, Sir Paul Reeves and Sir Hepi Te Heuheu