The residents did not have the money to produce expert evidence to dispute the mining company's testimony at consent hearings, she told Radio New Zealand.
"People just don't have the money to produce the international expertise to show that their story, their effects are more than anecdotal, and yet I get constant calls from local residents about what they're going through," she said.
"Already there have been effects from the Favona and Trio mines, but they weren't directly under people's houses. Now it's direct."
She acknowledged those who worked at the mine would be pleased with the decision, but said "the people affected by the mine will be devastated".
Ms Delahunty said Waihi was an economically vulnerable community, despite the millions of dollars of gold that have been mined from it.
"The rest of Coromandel is managing fine without huge holes in the ground and huge holes under people's houses. We have a sustainable agriculture, fishing, aquaculture and tourism economy and we believe the gold is better under the ground."
The consent would allow mining at a depth of around a third of a kilometre underground.
Commissioners appointed by Hauraki District Council to hear and determine the land use consent application by Newmont Waihi Gold to undertake mining works within the Golden Link Project Area (including the Correnso Underground Mine) have released their decision.
"The top of the Correnso ore body is approximately 130m below the land surface but the current proposal has the top of the mine workings at 157m below the surface," the decision said.
"The ore body extends to 430m below the surface, with the majority of the proposed mining to take place at a depth of 270m to 350m below the surface.
"The mine will be accessed by drives and a spiral decline from the existing Trio mine and Favona mine workings.
"The surface access point will be the existing Favona mine portal located near the processing plant."
A residents' group opposing the mine say affected homeowners feel short-changed by the decision.
Collette Spalding, spokeswoman for the Distressed Residents Action Team, which represents about 80 households, said mining should not be permitted under private homes.
"We've got no problem with mining, as long as it's their property,'' she said.
Ms Spalding said the mitigation offered by Newmont, through purchasing some properties and offering top-up payments to increase the purchase price of others, was inadequate.
She said the top ups were distorting the property market, and those selling homes were still likely to have to put in extra money to buy a similar property elsewhere.
There were already ongoing issues with seeking compensation for those affected by the existing Martha, Favona and Trio mines, she said.
The group had not yet had considered whether they would appeal against the decision, Ms Spalding said.
"But the few residents I've spoken to that are directly affected by this decision are not at all happy. They feel they've been short-changed.''
The Waihi East Ratepayers Group said they were disappointed with the decision, but would not make any further comment until they had a chance to go through the 86-page decision.
"We need to consider the decision and the conditions,'' said spokesman Terry Podmore.
EPMU assistant national secretary Ged O'Connell said the mine would provide continuity for mine workers as current operations wound up. "They'll be able to continue what they're doing, and I think there'll be a bit of growth out of it.''
He said the consent process was robust and had taken into account the interests of the business, residents and the environment.
"It's really important that there's a balanced and transparent process, and I think this one seems to have been.''
