The High Court has ruled Whanganui District Council's gang patch ban is unlawful.

The Hells Angels gang sought a judicial review of the ban, arguing the bylaw - banning the wearing of gang insignia in public in Whanganui - was invalid.

Justice Denis Clifford, in a judgment released today, said he had reached the same conclusion.

The bylaw did not specify which public places the ban applied, and was therefore invalid under national law, he said.

It was, in some circumstances, justifiable to limit freedom of expression but in this case the bylaw was invalid because the council had not considered the significance of limiting the gangs' right to express themselves, relative to the effect of the bylaw, Justice Clifford said.

However, he said a bylaw which more closely defined specified places could well be legal.

Whanganui district councillor Michael Laws, who was mayor when the bylaw was adopted in 2009, said the High Court was wrong and he would urge the council and police to appeal today's decision.

The council had expressly considered freedom of expression issues in its debate and resolved that the wider interests of the community had sway, he said.

However, the most important finding was that the court upheld the council's right to create a bylaw, Mr Laws said.

"It will be for my council colleagues to choose which course they will adopt but the gang patch ban is here to stay."