The council had signalled they were willing to compromise on their original plan but Ms Spiro felt the changes could not be seen as a compromise as most of the grass would be covered by the boardwalk.
Councillor Murray Guy was also unhappy with the council's decision and said he would be mourning alongside Ms Spiro at the ceremony.
"I've never supported the process and I've never supported the outcome," he said.
Tauranga mayor Stuart Crosby was made aware of the posters last night but said their presence would not bother him and would not put a damper on the ceremony.
"There were some reasonably strong views on the project so I'm not surprised," he said.
"They are entitled to have a strong view on it.
"That's part of the democratic process.
"As long as they are not defacing anything I'm quite relaxed about it."
Mount Maunganui Progressive Association chairman David Burnett said the group did not always support the boardwalk plan but were pleased with the final outcome.
"The decision that was made by the council was essentially a negotiated settlement done in public between the people who wanted to see change to their plan as opposed to what the council had proposed," he said.