The century-old Wahiawa dam, located in the north of Hawaii's third largest island, which is home to the state capital Honolulu, "is at imminent risk of failure," the local emergency management agency warned.

There is "potential life-threatening flooding" in areas below the dam, it added.

"LEAVE downstream area NOW!" it said, recommending that people carpool to ease traffic congestion.

Hawaii Governor Josh Green said torrential rains have caused "serious flooding" and brought water in the reservoir to dangerous levels, prompting authorities to issue the alert.

"Lots of people had to evacuate. We've evacuated the whole region now," Green said on social media.

Much of the US state, a remote archipelago in the Pacific Ocean, remains under a flood watch or warning, according to the National Weather Service which reported particularly dangerous conditions in northern Oahu.