The Maori Council has told the urgent Waitangi Tribunal hearing into water rights that a long-held belief by hapu and iwi of taniwha as guardian spirits of the waterways is strong evidence of Maori belief that they 'owned' the water.

A Treasury official says Maori rights in water and geothermal resources are a risk to the value of energy companies, and that it will be made clear to investors when Mighty River shares go on sale.

The Maori Council's Waitangi Tribunal water claim has served notice that the "taniwha" of Maori customary ownership has been awakened by the Government's partial asset sales plan.