Meanwhile, roading crews will head to Arthur's Pass this morning, after floodwaters swept away a bridge on State Highway 73.
Greymouth police were joined in the search in the Grey District by Fire and Emergency New Zealand crews, as well as members of the public.
The river runs past the Gloriavale Christian community.
The search started at 8pm and the person had not been found late last night.
A section of Goat Creek bridge near Otira was swept away by rising floodwaters
SH73 is the main connection between Christchurch and the West Coast.
A NZ Transport Agency spokeswoman said heavy machinery was ready to be moved to the site first thing this morning. Most of the machinery was in the area already and the roading crews would be ready to go at daylight, the spokeswoman said.
The bridge was replaced in 2010 when the old timber bridge was deemed to be unsuitable and nearing the end of its life.
Otira resident Jess Dempsey told the New Zealand Herald she and her husband had found the bridge just east of town had been washed out yesterday afternoon. She called the weather "absolutely terrible".
There were road snowfall warnings last night for Arthur's Pass (SH73), Porters Pass (SH73), Lindis Pass (SH8) and the Crown Range Road. Up to 10cm of snow was expected on the passes and about 1cm on the Crown Range. Heavy rain in the West Coast area yesterday closed highways and schools, led to Civil Defence activation in Westland and the 8.15am TranzAlpine train trip between Christchurch and Greymouth was cancelled.
- Additional reporting NZME
