An Alzheimer's sufferer who went missing from his Waikato home last night and was discovered early this morning partially submerged in a stream was lucky to have survived, police say.

A Ngaruawahia man, believed to be Doug Robinson who went missing from his Couch Street home at 5pm yesterday, was reported missing about 8.35pm when he failed to return from a walk.

He was discovered by police in a small stream near the bottom of Eagle Street, adjacent to the Waipa River, at 2.30am.

Police said Mr Robinson was known to walk about 6km twice a day, following walkways along the Waipa and Waikato rivers.

Both the police and civilian search and rescue squads were training in Kawhia, about 90km southwest of Ngaruawahia, when police were notified Mr Robinson was missing, and began the search.

Waikato Police Sergeant Phil Bell said it appeared he had fallen through a safety rail on a small footbridge and become stuck in the muddy water.

Mr Robinson told police he had been crying out for help but no one heard him, and the more he struggled the deeper he went into the water.

Police found him with one shoulder under the water.

Mr Bell said the man was "very, very fortunate" to have survived so long in the cold conditions.

The man was taken to Waikato Hospital suffering hypothermia.