The Government is warning completed Treaty negotiations could go back to the drawing board if we start charging for water.

It said the Labour Party's bid to place a royalty on water assumes it is owned by the Government.

Prime Minister Bill English said that could cause a lot of problems.

"They've now asserted ownership, that means ownership is going to be contested. We've spent nine years on this issue, we've never asserted ownership but we have worked with Maori on rights and interest because that's been a much more constructive way forward."

English said water ownership, no matter what's promised during a campaign, will be challenged.

"The Labour Party have bumbled into it. It's another half thought through policy, done on the fly because it looks politically attractive."