Hell's Angels have lodged an application in the High Court at Whanganui for a judicial review of the city's gang-patch bylaw.
Whanganui police estimate that the number of gang members in the area has fallen by 15% because of tougher policing and the patch ban.
Mayor Michael Laws says the police report is the first empirical data to show that the ban is working.
The lawyer acting for Hell's Angels, Steven Rollo, says however that the bylaw infringes fundamental freedoms.
He says the areas where gang patches and insignia can't be displayed are more extensive than Parliament allowed for when it passed legislation to enable the Whanganui District Council to pass the bylaw.
Mr Rollo also says the council was wrongly informed about matters of law, which undermines the validity of the bylaw.
More reporting gang activities
The bylaw, which came into effect last September, prohibits the wearing of gang insignia in certain parts of Whanganui.
Police say there have been 13 prosecutions since then, though
Inspector Duncan Macleod says the bylaw is not the sole reason for the success of the crackdown on gangs.
He says he has noticed more people coming forward to report gang activities.