The much-maligned Foreshore and Seabed Act 2004 has just this month shown its true colours. An agreement reached between the Government and Ngati Porou shows that, far from suppressing customary rights, the act is capable of giving them powerful effect.

While it preserves public rights of access to the water for enjoyment and navigation, it proposes to allow coastal-dwelling hapu exclusive authority and rights over a range of marine activities. These include the right to veto resource consents for activity that would harm the hapu's "relationship with the environment", which would seem to cover everything.

There will also be a legal instrument giving the hapu power to make bylaws under customary fishing regulations, and an environmental covenant that will insist district planning must conform to a Ngati Porou statement of sustainable management for the foreshore and seabed of its rohe (tribal territory).

While public access will be generally preserved, the hapu will be able to restrict or prohibit access to sacred places declared wahi tapu. And it will be able to carry out specified customary activities on public foreshore and seabed without needing resource consents.

The agreement has been three years in negotiation and much remains to be decided, including the protected customary activities and the coastal boundaries of the signatories. Indeed, their rights under the deed of agreement depend upon their proving that the hapu has been the exclusive occupant and user of land adjoining the claimed foreshore since 1840.

That proviso could be hard to meet, according to judges of the Court of Appeal in the landmark ruling that customary ownership of foreshore and seabed could be established. The irony of the legislation passed to preserve public ownership is that it may be a great deal easier to have a customary claim recognised in negotiation than by litigation. Now that the meaning of customary rights has been substantially agreed, the rights might more readily be granted.

It remains to be seen, though, whether the rights and authorities recognised in the agreement with Ngati Porou are sufficient to avoid conflict in practice. It is one thing for lawyers to draw fine definitions on paper, quite another for people at a beach, whether members of the hapu or recreational visitors, to know or care where customary exclusions begin and end.

The principle of customary possession, once accepted, will be more important than precise meanings in law. The East Coast agreement has now compromised public ownership of beaches and sea much the way that Treaty settlements have given claimants "guardianship" over mountains, lakes and parks.

The arrangement seems to work without conspicuous problems for trampers, campers, hunters and other recreational users of remote attractions. But beaches and bays will be a more visible expression of privileges resting on a pre-colonial heritage.

Four years ago, when the act was passed, the National Party's previous leader said, "Maori can be owners, managers and regulators at the same time. It is astonishing the Government could establish such a conflict-ridden model. It is an absolute recipe for disaster."

He has gone and the first deal has been done. Much detail remains to be decided, probably after the election, but the die seems cast; the coast in some places is going to be controlled by a tribal mandate. The mana thus acknowledged is enhanced when exercised with care and generosity. Maori know that; others may be surprised at how well this agreement can work - for everyone.