Emergency services used a thermal camera to track down a man found hiding in the Waikato River two hours after a car crashed overnight into an area flooded by the river's overflow.

A substance believed to be methamphetamine was found in the crashed rental car, while a 41-year-old gang member found at the crash scene had $1200 cash on him, police said.

A 19-year-old woman was also found lying face up in the water at the crashed car, with the 41-year-old man underneath her, trying to avoid detection.

The incident started shortly before midnight when police went to stop a car, police shift manager Senior Sergeant Andrew O'Reilly said.

The driver refused to stop and drove away at well over 100kmh. Searching police patrols noticed wheel marks on the grass near an intersection, and the car was found upside down in an area flooded by overflow from the river.

Firefighters and paramedics treated the pair found at the car before they were taken to Waikato Hospital by ambulance, O'Reilly said.

Police launched a rescue boat and arranged support from the Waikato River harbour master to put a second boat in the water.

Using a thermal camera provided by firefighters, a 29-year-old man was found hiding in the river, clinging to the bank about two hours after fleeing from the crash scene.

"We believe this person, who was wanted on a parole recall warrant, was the driver of the car. He was pretty relieved to be pulled out of the water, even though he faced arrest, and he was taken to Waikato Hospital for treatment for hypothermia," O'Reilly said.

"The thermal camera made all the difference. We had a police dog at the scene which kept trying to jump into the river and we thought it was being distracted by activity by the car upstream but the heat signature provided by the camera showed the dog was right and was trying to get into the water to get to the offender."

The 41-year-old gang member found hiding under the woman had been wanted on an arrest warrant for outstanding fines and the $1200 cash he had on him when arrested had been forfeited to the courts.

Police inquiries would focus on the substance believed to be methamphetamine recovered from the car.

- © Fairfax NZ News