Wanganui MP Chester Borrows says Hell's Angels is unlikely to be able to convince anyone it is a club rather than a gang.
The Prohibition of Gang Insignia Act enables Wanganui Council to ban gang patches in public places.
The bylaw is yet to come into force, but members of the Hell's Angels Motorcycle Club plan to fight it, saying they are a motorcycle club, not a gang.
Mr Borrows, a National MP, sponsored the legislation that allows the by-law.
He says Hell's Angels members will have a hard fight to prove they are not a gang, adding that their argument is rather like saying Mt Eden Prison is a boys' home.
Hell's Angels members in Wanganui have hired a lawyer to help them argue their case before a council sub-committee on Friday.
Lawyer Steven Rollo, of the firm Lane Neave in Christchurch, who is representing the Hell's Angels, says a bylaw cannot be based on a "popularity contest" and the council has no grounds to justify the move.
"Regardless of what people's personal views are of these organisations, the council has to gather sufficient information to justify the bylaw. We say they're not doing that.
"We're really concerned to see that the Wanganui District Council bases its decision on facts, as opposed to (Wanganui Mayor Michael) Laws' rhetoric," he said.
Mr Rollow says his clients have been denied the right to participate in the process.
Wanganui district councillor Rob Vinsen also opposes the ban on the grounds that it does not serve the intended purpose and gives Wanganui an unjustified reputation as a city with a gang problem.