The council, which owns the port, could not issue a binding management directive to the port company, he said.
The protest group says the port company is incrementally expanding into the Waitemata Harbour, and its actions are being sanctioned by its owner, Auckland Council.
The group spokesman is Ponsonby music teacher and boatie Michael Goldwater. Other members include boating groups, architects, politicians and business organisations.
The extensions would unnecessarily narrow an increasingly congested harbour and block the view to the outer harbour from Queens Wharf, Mr Goldwater said.
In December, council staff granted resource consent for two wharf extensions nearly 100m into the harbour.
There was no need for the consents to be notified, and officers did not tell councillors or the public.
Mr Goldwater said that in 2013 Mr Brown promised a study of the social, cultural, environmental and economic effects of Ports of Auckland's place in the city.
"We still wait for that study. Until that is done, the Bledisloe Wharf extensions must be stopped," he said.
Eleven days ago, Mr Brown, deputy mayor Penny Hulse and a slim majority of councillors voted 12-10 behind closed doors to insert rules in the council's Unitary Plan that will make further reclamation of the harbour for port use easier.
They rejected a last-gasp plea from Warehouse founder and Kiwibank New Zealander of the Year Sir Stephen Tindall to use all their powers to stop the wharf extensions - "the first step to further reclamation".
Last night, Mr Brown said he had initiated the first elements of the full study.
He said the new rules being proposed by the council for the Unitary Plan were more restrictive than the existing rules, and could come into effect next year.
A ports spokesman said the company needed the wharf extensions because the volume of bulk cargo - handled at Bledisloe Wharf - had doubled in five years and ships had got bigger.
"We have got much more efficient in our cargo handling, which reduces our land needs, but you can't put a big ship on a short wharf."
The spokesman said there had been two studies in three years showing the port needed to grow.
"Another study isn't going to change that," the spokesman said.
"We have put off the decision as long as we could but the need is now urgent."
Stop Stealing Our Harbour
• New group joins outcry against wharf extensions.
• Group calls on Auckland Council to halt extensions.
• Ports of Auckland plans to start wharf extensions next month.
• Mayor Len Brown says he cannot intervene to stop work.
• Port says wharf extensions first stage to reclaim 3ha of seabed.
Aucklanders are being urged to join the campaign on: facebook.com/stopstealingourharbour