The Maori Party says Labour has to sort out who owns freshwater before its begins taxing it.
"You can't start charging for something you don't own," co-leaders Te Ururoa Flavell and Marama Fox said in a statement tonight.
Labour had said it would respect iwi rights in relation to water.
"If that is the case, then those rights and interests need to be determined before anyone starts taxing water.
"Any discussion around water rights, interests, management, quota, ownership, pricing or quality must involve hapu and iwi and the same goes with any potential land tax."
Labour has said it will introduce a water tax on commercial consumption of water, which it is calling "a clean water royalty."
It has said it will talk about the rate at a summit after the election but irrigation charges would be around 1c to 2c per 1000 litres and bottled water would be charged on a per litre basis.
It has also said some of the royalty would be given to iwi in a Treaty of Waitangi settlement and the rest to regional councils to clean up waterways.
"Labour has promised to hold a roundtable discussion in its first 100 days to set the price for water, but how can that be done when issues around rights and interests remain unresolved."
Labour's position is that everyone owns the water but it accepts the Waitangi Tribunal finding that Maori had rights and interests in water in 1840 equivalent to ownership and which were covered by the Treaty of Waitangi.
National's position is that no one owns the water. It opposes a water tax.
Treaty Negotiations Minister Chris Finlayson says Labour's position this election overturns 25 years of treaty settlement policy of both parties.
He has also said that the Government applying royalties to water was an assertion of ownership which would attract a counter claim by Maori that they owned the water.
He says water royalties would open a Pandora's box and could lead to full and final treaty settlements being reopened.