Mr Donnelly said the council had made a "fundamental administrative flaw", albeit unintentionally, and was misleading the public about the process currently under way.
The council wasn't reviewing its 2006 bylaw, but was actually consulting on a new bylaw.
"In these circumstances, a fresh consultation process under section 82 of the Local Government Act should take place."
Watchdogs spokeswoman Leonie Exel, of Russell, said a bylaw adopted by the council could be challenged by judicial review.
"But what we're really trying to say to the council is, stop what you're doing, think, get external legal advice, and get this right," she said.
The ChenPalmer opinion had been paid for by crowdfunding from its members, but a judicial review would cost a lot more, she added.
Mayor John Carter, however, said the council had sought its own advice, from law firm Simpson Grierson, which said the 2006 bylaw had to be reviewed after 10 years.
That time frame had been met when the council resolved to review the bylaw on June 15, 2016.
Public consultation began in November 2016, and final deliberations were now under way.
"Until the new, amended bylaw is adopted, the 2006 bylaw remains in force. Independent legal advice confirms this position," Mr Carter said.
The original bylaw was adopted in 1997. Meanwhile, councillors will continue their deliberations of the record 1215 submissions on November 27, and are due to put the bylaw to the vote on December 13.
The Watchdogs also claim the council does not have authority to pass bylaws to protect wildlife. That, according to ChenPalmer, was the role of the Minister of Conservation under the Wildlife Act 1953.