Southland District Mayor Rob Scott launched a reorganisation initiative last year seeking to combine the four councils in Southland into two unitary authorities.
Mr Scott says it will bring improved governance, efficiency and community outcomes.
But Gore District Council chief executive Debbie Lascelles has cast doubt on these claims.
The proposal is being investigated by the Local Government Commission and the commission has asked each council to give its thoughts on the amalgamation proposal.
Responding to Local Government Commission chief executive Penny Langley in a letter, Ms Lascelles expressed concerns over the scale and viability of the project.
Ms Lascelles wrote the potential benefits were overriden by the complexity of the two proposed authorities, and would be a costly endeavour.
"Any small savings that would be gained through the reduction of governance and senior staff in the amalgamation are likely to be eroded by the addition of a second unitary authority."
Gore is collaborating on a shared Three Waters council-controlled organisation with the Clutha, Central Otago and Waitaki District Councils, and shared water schemes with Clutha.
If the amalgamation went through, Gore would withdraw from these agreements in line with the Invercargill and Southland councils, which are not a part of any co-operative agreement, the letter suggests.
"From a practical perspective, it may not be feasible or desirable for us to join with councils who are not willing to do the work and make efficiencies to their waters programme," the letter said.
Ms Lascelles also cited a Department of Internal Affairs investigation into the most financially efficient entities, and said its research suggested a population of 200,000 was required before significant economic benefits were realised.
A potential population of 100,000 of the new unitary authority could not be considered financially viable for the current model.
"We propose that a much larger amalgamation would be required to justify the considerable cost that an amalgamation would require."
The Gore District Council’s work on the shared water entities and other endeavours had taken up staff capacity as well, and she was not willing to add extra to both their plate and the budget to investigate the potential for amalgamation, Ms Lascelles wrote.
"Our finances are already stretched. We account for every single dollar in our budgets, and likewise with our staffing capacity.
Mayor Ben Bell said, in a statement, he fully agreed with Ms Lascelles and the proposal needed a broader scope to justify its existence.
"This will be a once-in-a-generation decision and needs to be done correctly. An amalgamation of this size would only solve half the perceived problem.
"Why waste money tinkering around the edges?"
Mr Bell said the Gore District Council’s focus was on the water entities and the confirmed collaboration between councils — not potential mergers.
"As the only Southland council willing to work with others on Local Water Done Well, this amalgamation conversation is a distraction."
The letter will be presented to the Gore District Council at a meeting on a later date.
Mr Bell was in Invercargill last night, where he, Mayor Rob Scott and Invercargill Mayor Nobby Clarke were invited to a closed-doors meeting to debate amalgamation, among other topics, by agricultural group Livestrands. The media and members of the public were not invited to the meeting.
