“Is it legal to roll a joint in a public area such as a park? ”
It is legal to possess cannabis in a public place, and there is no law that would appear to ban an adult from any kind of cannabis-related prep or cleaning in a public place (like a park).
However, there are very particular laws around where you can legally consume that joint, once it has been rolled. Unfortunately, a public park is not one of them. For the full law on this area, check out the "Consumption" section of the NB Cannabis Control Act.