"From a national-needs perspective, we don't need both the [Waitaki River] north bank tunnel scheme and Project Hayes wind generation," he told an Environment Canterbury hearings panel on Thursday.
Meridian's Project Hayes, a 176-turbine wind farm on the Lammermoor Range in Central Otago, is at present before the Environment Court.
Meridian's north bank tunnel concept (NBTC) is a hydro-electricity scheme with a 34km-long tunnel and a single powerhouse on the north bank of the Waitaki River between the Waitaki dam and Stonewall, west of Ikawai.
Mr Mitchell was giving evidence on behalf of the Waitaki Protection Trust and Waitaki First, both of which are opposed to the north bank tunnel concept scheme, at a resumed hearing to make a decision on four resource consent applications by Meridian for water to power the scheme.
At a hearing last year on Meridian's applications, Mr Mitchell had said north bank tunnel concept scheme was not needed and did not figure in electricity planning by the Electricity Commission and Transpower, contradicting evidence given by Meridian.
On Thursday, Mr Mitchell maintained that stance, despite evidence from the commission and Transpower that they had no responsibility in determining what new electricity generation projects should be built and where.
Mr Mitchell said other electricity generation projects had lower costs for the electricity they produced.
Transpower's annual planning report "clearly indicated" the Clutha River was the priority target for hydro development.
West Coast and north Canterbury proposals were more desirable because they would generate in an electricity-short area.
Other proposals, such as wind, were more economic and more likely in terms of location, needs and (resource) consenting.
"These schemes will, in all likelihood, proceed before the north bank tunnel concept would be considered and their development will threaten the ultimate viability of NBTC.
"In terms of growth in demand, they push the possibility of NBTC so far out into the future that any consent now would need to allow for an excessively long start period," Mr Mitchell said.
Granting resource consents now for NBTC would lock-up resources from other users before the economic viability of the scheme had been established with some degree of certainty.
Every indication was the scheme was not the most economically efficient and unlikely to be built for a considerable period until after better alternatives.
By that time, other technological developments would undermine viability further.
"The NBTC consent applications are premature and not in the national interest," he said.
