Mohaka Rafting, who have been operating since 2009 and take about 2000 people a year down the river, had an "exemplary safety record" prior to the incident.
"I understand that the last fatality on the river itself was back in 2010/11, but that was a private trip and absolutely no connection to Mohaka Rafting," the spokeswoman said.
Police received a report that the person had gone underwater while rafting on the river, near Kotemaori, in Wairoa, about 1.45pm on Saturday.
A police spokeswoman confirmed the person was pulled from the water, but was unable to be revived and died at the scene.
The Mohaka Rafting spokeswoman said that all those at the company are "devastated" and the company's priority now was to support their guides and the rafting group.
"It was a devastating incident and everyone involved is in pieces," she said. "How this happened will all be part of the investigation Maritime New Zealand will do.
"The guides are all extremely familiar with the river and with the white water section in particular. Safety is absolutely paramount and the guides' number one priority at all stages of the trip."
The spokeswoman added: "The deceased were not foreign visitors – the group were New Zealanders."
A police spokeswoman added: "I understand a rahui has been put in place by local iwi at the river mouth, which will be in effect for three days."
Police said they are working to establish the circumstances of the fatal rafting incident.