A major court case which observers say pits Maori spiritual belief against the need for a secure power supply starts in the high court on Monday.
Genesis Energy is seeking to overturn an environment court ruling on rights to water from the Whanganui River. The Tongariro power scheme provides around 5% of the country's electricity and the water it uses comes partly from the river.
Whanganui iwi claim it was taken without consent.
"We have no choice but to ensure the wellbeing of our river, the restoration of the waters of our river," says Ken Mair from the Whanganui River Trust Board.
Last year the iwi went to the environment court which ruled the diversions were culturally unacceptable to Maori and Genesis Energy's water rights were cut from 35 to 10 years.
Owen McShane from the Centre for Resource Management Studies says that is very scary for anyone who is thinking of investing in hydro power.
"Now we have a sort of precedent that says well rivers in New Zealand have a life force and generating stations take that life force away, and that's going to upset Maori who believe in that," says McShane.
Genesis is appealing against the decision and observers say if it loses, investors will go elsewhere.
"You can't pay off and justify an investment in a hydro scheme when you've only got a 10 year fuel supply," says McShane.
Electricity demand is growing by about 5% a year and not enough new generators are being built to ensure supply.
Alan Jenkins from the Electricity Networks Association says the principal objective of having enough power to meet demand is steadily being eroded.
"It's very hard to invest in coal, nuclear's a sort of four letter word...hydro is suddenly becoming too hard...what's left?...we can't do everything on windpower," says Jenkins.
The Whanganui River appeal case is set down for three days.