An elderly man with dementia is lucky to be alive after a police helicopter found him in a water-filled drain near Waihi in the western Bay of Plenty overnight.

Bryan Thomas Warrander, 76, had been missing from his Waihi home since Wednesday night.

He had been seen by a motorist pushing his bike down the middle of SH2, near Waihi, heading to Tauranga shortly before midnight.

Using an infrared camera, Auckland police helicopter Eagle found Mr Warrander semi-conscious about 9pm.

He was hypothermic and taken to Tauranga Hospital by ambulance, said Sergeant Dave Litton from the Waihi police.

"We believe it was extremely fortunate he was found when he was, which is a testament to the skill and professionalism of the search teams involved."

He thanked the public for their help in the search, after 50 people formed 15 search teams.

Volunteer searchers came from throughout the Coromandel and Waikato.