The Mt Coot-tha Botanic Gardens volunteer guides have been ordered not to discuss the zipline, even in their own tearoom.
Former guide Vicky Henry said she decided to resign so she could speak up about her concerns.
She said under their code of conduct they were not allowed to mention their views on the project to visitors, but most guides were strongly opposed to it.
“When we had our dinner for the guides last month our president expressed our concerns to the Lord Mayor — the room exploded with applause,’’ Ms Henry said.
“We are still not certain if they will have to broaden the paths to allow shuttle buses to use them.
“But they will have to remove a lot of plantings in the Australian Native Plant Zone for the landing stage and bus turn around.
“This is one of the premier botanic gardens in Australia and a lot of effort and expense has gone into making it look as good as it does.’’
She was also concerned about the impact of people moving through the gardens at night.
The council did not offer a response to any of Westside News’ questions about Ms Henry’s claims.