For what’s happening at Waitangi see 2016 Waitangi Day Celebrations.
There’s a lot of Waitangi Day related events around the country. You’ll have to check locally if you’re interested. There’s a few low key events planned for Dunedin.
NZ History has information on the past and present of Waitangi Day:
Waitangi Day is for whatever you want it to be. However you do it have a good one.