Council documents state that bylaws have to be reviewed every 10 years. The current review started in 2016.
However, ChenPalmer special counsel Leo Donnelly said any new bylaw created under the Local Government Act had to be reviewed after five years. After that reviews had to take place at 10-yearly intervals.
A new bylaw not reviewed within five years would lapse once another two years had passed, he said.
That meant the current Dog Control Bylaw, made by special order in 2006, had been revoked on September 18, 2013, and prosecutions since then could be unlawful. Even in the best case the current bylaw had been revoked on September 18 this year, 12 years since it was passed.
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The council wasn't reviewing its 2006 bylaw, which he said had been revoked, but was instead consulting on a new bylaw.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 if the council went ahead and passed the bylaw, it 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.''
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 timeframe had been met when the council decided to review the bylaw on June 15, 2016.
Public consultation started 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,'' he said.
The original bylaw was passed in 1997.
Excel said the ChenPalmer opinion had been paid for by crowdfunding from its members. A judicial review, however, would cost a lot more.
Councillors will continue their deliberations of the record 1215 submissions on November 27 and are due to put the dog bylaw to the vote on December 13.
The Watchdogs also believe, based on an earlier legal opinion about a cat bylaw in Wellington, that the council did not have the power to pass bylaws to protect wildlife.
That was the role of the Conservation Minister under the Wildlife Act 1953, according to ChenPalmer.