Coutts said there was not enough public interest in the findings to outweigh the need to withhold them.
He said the accident happened around 10am on a day of sun and intermittent heavy rain. The kayaker had left Tieke Kainga and Marae about 7am.
He was alone and heading downstream towards Pipiriki as department staff headed upstream in a jetboat, to do routine maintenance on the Mangapurua Track.
The rain was heavy and visibility was poor when the jetboat collided with the kayak. The staff recovered the man from the water and contacted emergency services. He was taken to Whanganui Hospital.
The names of all concerned are withheld to protect their privacy, Coutts said.
He could not say how the injured man is now.
No staff lost their jobs as a result of the accident, he said.
The department put a 90-day "stop" on work on the Whanganui River from April 23.
During it most of the 23 staff in the Whanganui District worked from the Whanganui office - but no long term changes to their work place are planned.
It was a chance to refresh health and safety practices, introduce new software and add a new layer of assessments and approvals before work is undertaken.
As a result, some work has been delayed.
The Whanganui staff care deeply about their steep and remote rohe and have lots of skills and experience, Coutts said.
"It is important the department supports them to operate safely in such a challenging environment."