Resource commissioners gave final approval on Tuesday for a $280 million hydro power scheme on Marlborough's Wairau River.
However, Trustpower must meet conditions, set out in a 300-page document, relating to river flows and ecological and environmental considerations.
Trustpower spokesman Graeme Purches says some of the company's legal advisors have said this is the most comprehensive decision they have seen come out of the Resource Management Act.
"There's been no stone left unturned, in terms of the detail, and in terms of traversing the evidence that was presented and the validity of that evidence," he said.
The project, which involves 50 kilometres of canals, was opposed by environmentalists during a six month hearing.
Mr Purches says opponents may yet appeal to the Environment Court, which could delay the project for another year.
Trustpower says its board may consider a schedule for the project before the end of the year, if there is no Environment Court appeal.
Fish and Game to appeal
Fish and Game's Nelson manager, Neil Deans, says the canal-based project will drastically reduce the Wairau river's flow. He says the scheme contravenes Marlborough's district plan.
Fish and Game says it will go ahead with an appeal to the Environment Court against the scheme.
A resident in Marlborough's Wairau Valley says Trustpower has told her it may acquire her land compulsorily if she will not make it available for a power scheme.
Alison Parr says she and a few other residents are refusing to allow Trustpower to build a canal on their property.
Trustpower says legal powers to acquire land may be available, but it is continuing to negotiate with landowners.