The letter said a meeting would be held with representatives from the Taupo Fishery Advisory Committee on July 7 at the Taupo DoC office to discuss the issue.
Asked about the contents of her letter, Ms Alexander-Turia referred the Herald to the Office of Treaty Settlements.
A spokesman for the office said: "The agreement in principle does not allow for commercial trout farming. The use of the raceway is to raise trout for cultural purposes only. We believe this arrangement will enhance the educational and cultural role of the Tongariro Trout Centre."
However, Tony Orman, executive member of the New Zealand Federation of Freshwater Anglers, said the proposals in the letter "set a dangerous move towards allowing commercial farming of trout, an issue which over the decades has been strongly resisted by the trout fishing public".
"It's the thin edge of the wedge. Successive governments have turned down strong lobby efforts by commercial exploiters to allow trout farming," said Mr Orman, author of nine books on trout fishing.
"Trout farming would put at risk New Zealand's world-famous wild trout fisheries, which attract many overseas anglers and earn New Zealand multi-millions in tourism. It would be lunacy to endanger that."
Mr Orman said it was incongruous that DoC was proposing "a form of trout farming" when DoC received anglers' licence money for Taupo to represent the interests of recreational licence holders.
Bryce Johnson, chief executive of Fish & Game New Zealand, also questions the constitutional basis for an inclusion in a historical Treaty of Waitangi claim when trout were not introduced to New Zealand until the late 1800s, long after the Treaty was signed.
"Successive court cases have explicitly confirmed that trout, as an introduced species for sports fishing, are not a consideration under the Treaty, so where is the legal basis for this proposal?" Mr Johnson asked.
Allan Simmons, a Taupo-Tongariro fishing guide for nearly 30 years and president and outdoors spokesman for United Future, said: "This current proposal looks like a devious attempt to bring in trout farming under the guise of Treaty settlements and utilise facilities that have been funded by anglers' licences."