In principle, dogs must always be on a leash in the wild. This is because the dog can be a stress factor for nature's wildlife and to the nuisance of other outdoor guests.
During the summer season, from April to September, dogs must be on a leash on the beach. During the winter months, from October to March, you may leave your dog loose on the beach, but only if it does not endanger other people.
On some beaches it is forbidden to bring dogs - always notice a local signage.