On Thursday, Urban Auckland lodged papers with the High Court at Auckland seeking a judicial review of the extension.
It says the resource consents were granted unlawfully.
The port company gained consent for the structures under the old regional council rules which did not require it to notify the public of the application.
A first hearing for the case will be on April 16.
The legal action comes alongside a request from Auckland Council to the ports to halt work on the extension until a wide-ranging port study is done.
Auckland Council chief executive Stephen Town has written to ACIL warning the ports had not "engaged sufficiently with key stakeholders" in relation to the extensions.
ACIL, the council controlled organisation which manages the council's major investments including the port, met to consider the letter on Thursday, and in turn wrote another letter to the POAL board.
Neither ACIL chief executive Gary Swift nor ports chairman Graeme Hawkins would reveal the content of the letter.
But Mr Hawkins said the port board would hold a meeting next week to discuss its response.
With regards to the full-page advertisement, Mr Hawkins said it was simply an extension of the campaign POAL had been running to tell Aucklanders about what it was doing.
The campaign was in response to criticism from the council that it hadn't informed people as well as it should have, he said.
The ad is headed "Passion fuels everything but the facts" and says "while everyone is entitled to an opinion, it is also important to be clear about the facts".
Stop Stealing Our Harbour spokesman Michael Goldwater said the port should have been communicating with ratepayers well before now.
He said: "I would question why they are taking out a full-page ad and using ratepayers' resources on this."
