By what authority do Maori groups impose restrictions on the rest of the community?

The other day, I noticed a report in the media that Far North hapu have imposed a rahui on taking scallops from the Whangaroa Harbor. The ban on taking scallops extended across the whole harbour, and was expected to last three years.

Another Maori tribe imposed a similar ban on taking shell-fish in parts of the Coromandel last year.

After the serious loss of life caused by the eruption of White Island more than a year ago, yet another tribe placed a rahui on the Whakatane waterfront, and erected a temporary barrier stopping access to it.

A few years ago, a tribe placed a rahui on parts of the Waitakeres to help protect kauri trees from a disease.

When somebody loses their life in a river, or at a beach, it is common to see a tribe declare a rahui over the relevant body of water.

Does the rest of the community just abide by those restrictions out of goodwill? It seems bizarre to have a government department formally charged with conserving our natural resources while simultaneously allowing small tribes to invent their own rules and require the rest of the community to abide by them.