A letter signed by prominent Aucklanders has been sent to Auckland Mayor Phil Goff and other councillors opposing the Queens Wharf extension.
Goldman Sachs' boss Andrew Barclay, Barfoot & Thompson director Kiri Barfoot and SkyCity entertainment chairman Rob Campbell are among those opposed.
Auckland Council's developing arm, Panuku, proposed the extension to allow large cruise ships to berth.
The $10 million 90-metre-long concrete extension, which is called a dolphin structure, would run outwards into the Waitemata Harbour.
The letter, which was to be signed and sent overnight, highlights concerns about the Waitemata Harbour's health which has already been impacted by Auckland's development.
"This incremental attack on our beautiful harbour has to stop," the letter reads.
"The decision to extend Queens Wharf, which will cost ratepayers more than $10 million, is based on an economic report that has been discredited by two of New Zealand's leading economists.
"We urge you to oppose the sale of our harbour for a few extra dollars, most of which will accrue to foreign-owned shipping corporations."
However, Panuku and Ateed cruise project manager John Smith the decision to extend the wharf is of great benefit for the entire Auckland region.
The industry says it's worth about $181 million to the Auckland economy from services used by ships while they're in port as well as the passengers visiting the region.
"In really simple terms we're saying, 'we'll spend some money up front, you'll bring some boats in and pay us to use that facility and then all the people who get off those boats will come and spend money'," Smith said.
Meanwhile, Goff said he remains supportive of the extension as the cruise ship industry continues to benefit the local economy.
"I expressed my support to ensure the cruise ship industry continues to deliver significant economic benefits and jobs in Auckland," he said.
"I will not comment further while the matter is following a statutory process by independent commissioners and the Environment Court.
"These bodies will make a decision based on the evidence which is objective and impartial."
Goff did, however, acknowledge the need to protect the harbour and said the council is investing "significantly" to improve its environment.
A protest opposing the proposal is planned by Stop Stealing Our Harbour at Queens Wharf on Sunday at 3pm.
Stop Stealing Our Harbour spokesman Michael Goldwater said the group wanted to maintain both the harbour and the wharf for public use for future generations.
"The 90m extension will remove a sizeable portion of the Waitemata Harbour from public use and further industrialise the 'People's Wharf'," he said.
"We want to save our harbour for future generations and celebrate a waterfront that will accommodate the commercial, environmental and social opportunities of a growing city.
"We are calling on all Aucklanders to join us to send a very loud message to Mayor Phil Goff and his council."