Hamilton mayor Julie Hardaker says people lobbying to legalise marijuana were offered alternative venues for a forthcoming protest, but instead are choosing to defy a ban and rally in a central city park.
Hamilton City Council says the planned rally, on 3 May, violates the city's smoke-free environment policy.
The National Organisation for the Reform of Marijuana Laws (NORML) is planning the protest and spokesperson Gary Chiles says attendees won't be smoking.
"It's a protest against the prohibition of cannabis and it's not promoted as a smoking event it's promoted purely as a political protest."
He is encouraging people to defy the ban and come to the rally.
It is not unusual for legalise marijuana supporters to take to the parks on international pro-cannabis day, or J-Day.
So the organisers of the Hamilton J-Day say it was a surprise to them when the council told them they weren't welcome at Lake Domain Stage.
Hamilton is one of nine New Zealand cities hosting similar demonstrations.
Mr Chiles expects over 100 people to defy the ban.