Aramoana Conservation Group spokesman Bradley Curnow believed it was not possible to build a wharf in the area without threatening the population of endangered sea lions.
''We are just really afraid that if there aren't some controls put on a new wharf, then the sea lions just won't haul up there,'' he said.
''They will lose their habitat, they will lose their home.''
He also believed the public had been kept in the dark about the full scale of the plans, which involved ''bulldozing a road'' to the wharf.
The group was confident it could stop the proposal, as it needed to be approved by the Department of Conservation, which controlled the land the wharf was on.
The group would be raising its concerns at a council meeting next Monday.
''We are just going to tell our councillors that they need to be careful of the environment when they make their decisions and also letting them know that we are not all 100% behind the wharf in Aramoana.''
Aramoana League member John Davis said he was ''disappointed'' the group had come out against the proposal.
It had not kept the public in the dark over the proposal and there were no plans to build a road to the wharf, Mr Davis said. The wharf would not threaten the sea lions in the area and would instead give people a better opportunity to see them in their natural habitat, he said.
He was keen to point out the old wharf could still be used today if it had been maintained by the council as it should have been.
Despite the setback, Mr Davis said the group had not given up on its plans.
The group had recently received a more detailed cost estimate of the project from Port Otago, which put the total cost at $150,000, up from the initial estimate of $100,000.
The league would be making a submission in favour of the proposal as part of the council's draft annual plan process on May 8.
Doc coastal Otago manager Robin Thomas said he was not in a position to comment on the plans as the organisation was yet to see a ''detailed proposal''.
Doc's response would depend on various factors, including how the wharf linked to the land, he said.
The Dunedin City Council's community development committee last year decided to defer a vote on demolishing the wharf until the options for its future were investigated.
