A 38-year-old woman has died after being swept away while riding a horse across the Ruamahanga River this afternoon.

The woman was riding with a friend near Matapihi Road, about 5km north of Masterton, at 2pm when she attempted to cross the swollen river, Masterton police said.

The horse fell and the woman was swept away.

Her riding companion tried to help her and then sought help from a nearby friend.

The friend searched and found the woman downstream.

She was taken to an island but could not be revived.

Emergency services and the Westpac rescue helicopter responded.

Police said her death would be referred to the coroner.