Masterton police told Horsetalk Bain, 38, died when her horse fell as they tried to cross the Ruamahanga River, just north of Masterton.
Police said the river was flooded and dirty at the time due to recent rain in the Tararua Ranges.
Bain, winner of several Qantas Awards while working for the Dominion Post newspaper in Wellington, was later political editor of the Sunday Star Times.
For the last three and a half years she worked as communications manager for Forest & Bird.
Bain, who was with at least one companion, was found downstream after the accident, which occurred around 2pm. Efforts to revive her proved unsuccessful.
She was a popular member of the Horsetalk forums.