Corey Stephens went up the Wanganui River towards Hunters Hut last Tuesday, and was expected to return on the Friday.
West Coast Search and Rescue co-ordinator Sergeant Sean Judd said on Thursday the police search team was looking at the lower reaches of the river and along the coast line.
"We know Mr Stephens was intending to head to the Hunters Hut. We're quite certain he didn't make it," Mr Judd said.
"He was also carrying a ... beacon with him and we knew that he carried that on his person, to avoid getting separated from it, and that hasn't been activated either. So all those things together, our optimism is certainly dropping."
Mr Stephens' name was not in the hut book, he said, adding that the man's family was finding it tough.