"We are against any extension into the harbour," said Goldwater, whose group was behind large public protests against wharf extensions into the Waitemata Harbour for port use in 2015.
Asked what action Stop Stealing Our Harbour would take to sink Goff's plan, Goldwater said "we will cross that road when we get there".
He is waiting for a technical paper on the options that councillors will use to make a decision on a preferred site on Thursday.
Urban Auckland spokeswoman Julie Stout said the group of urban designers and architects would also intensely resist a 50m extension.
She believes a third option put on the table after Economic Development Minister David Parker visited the waterfront on Sunday that has dispersed options around Wynyard Quarter could be made to work.
She said it could work towards the end game of Wynyard Wharf becoming a peninsula park available for America's Cup and other events like triathlons without having to build a big wharf into the harbour.
Goldwater said Stop Stealing Our Harbour is excited about the next America's Cup, and wants it hosted in Auckland.
"As Aucklanders don't want their harbour to be perpetually whittled away, the cup infrastructure must be accommodated using existing assets and does not require the continued erosion of the Waitemata Harbour," he said.
Stop Stealing Our Harbour believes the concepts proposed by Auckland Council are not in the best long-term interests of the city and the planning and analysis behind them is flawed.
"The America's Cup should be used to accelerate the continued redevelopment of waterfront, transforming negatives into positives," Goldwater said.
Hosting the America's Cup would boost New Zealand's economy by up to $1 billion - and create up to 8300 jobs, according to a report released today by the Ministry of Business, Innovation and Employment (MBie).
The key findings its High Level Economic Assessment Evaluation report for the 36th America's Cup include an estimated benefit of between $600 million and $1b to New Zealand's economy from 2018 through to 2021.
MBie estimates hosting the event would also create between 4700 and 8300 jobs.