J-Day is an R18 event which will be roped off for safety.
It will feature documentaries on medicinal cannabis; health stalls; and live entertainment.
Scarlet-Rose said organisers were not encouraging people to bring along cannabis but, if they did, that was their choice.
"If people want to bring stuff along I'm not going to say no but there will be no sale of it at the event," she said.
Whangarei District Council chief executive Rob Forlong said the council gave permission for the event to be held on council land but said it did not tolerate illegal activity and that was pointed out to organisers.
"We like to know what they will be promoting and how, so that we can point out that it must be a legal purpose, that it is a public space and that we have a responsibility ensure members of the public (in this case loop users, dog walkers, BMXers and skate bowl users and neighbours) won't be adversely affected," he said.
Mr Forlong said the council had good relationships with the police and other CitySafe partners.
Scarlet-Rose acknowledged there may be a police presence and she was worried there would be arrests.
"I'm hoping it is calm and if police turn up that's fine, they're well within their rights," she said.
Police were unable to respond by edition time yesterday.
Scarlet-Rose is a facilitator of Green Aid, an organisation which helps people who need cannabis for medicinal purposes to access it.
She said it was a risk for her to do that but she wanted to help people.
"There's a lot of people who are not happy it's illegal. There are people who really need it. It's a process of opening people's eyes to the fact they are not criminals or druggies. We need to take the money out of the gangs."