Māori signed a treaty with the Crown in 1840. Over the years it has been broken again and again and again. Since around the 1970's and more so since the 1990's there have been settlements to a number of Iwi (tribes) for breaches and land confiscations. Mostly this involves some monetary compensation, some (small amounts) of land back, rights to fishing quota and other natural resources. There has also been a move to live up to the terms of the treaty, article 2 of which kept the Iwi's rights to Rangitrirtanga (loosely - control) over their land, their waters and their taonga (loosely - treasures) by legislating for greater consultation rights (and in some cases co-management) of resources etc...
There is a (large) minority of people that don't like this, and want to unilaterally redefine the 'principles' of the treaty in legislation to become an essentially meaningless constitutional document, under the guise of equal rights'. I would like to think that the right wings principles of upholding the sanctity of a contract, and wanting to change a treaty or contract by good faith negotiation by both parties would win out, but sadly the racism and worry of someone else getting something that i'm not tends to win out.
That doesn't even go in to the place that Māori have in society, with poorer educational outcomes, poorer health, greater poverty, more incarceration, and less so now, but massive discrimination in the past including loss of language by the prohibition of the use of Te Reo in schools (not just in class). These people are against any measures to work to reduce any of those stats.
The party introducing the legislation received 8% of the vote, but went in to the Government in coallition, and insisted on the bill getting through to this stage as part of the agreement. The other parties in Government have committed to voting against it in the next stage.