Emergency sirens blared along Oahu's North Shore, where rising waters damaged homes in a community world-renowned for its surfing.
Honolulu officials told residents at 5.35am on Friday to leave the area downstream of Wahiawa dam, saying it was "at risk of imminent failure".
There were no immediate reports of deaths or injuries, but firefighters and lifeguards on jet skis were searching flood waters for people who had been stranded.
Officials have been watching dam levels since a storm last week dumped heavy rain across the state, which led to catastrophic flooding that washed away roads and homes. After the worst of it, a similar but weaker storm was forecast to bring more rain through this weekend.
Most of the state was under a flood watch, with northern Oahu under a flash flood warning, according to the National Weather Service, which reported "widespread life-threatening flash flooding" that's "extremely dangerous" particularly in Haleiwa and Waialua.
One shelter at Waialua High and Intermediate School is being evacuated because of flooding. There were about 185 people and 50 pets who had sought shelter there but are being bussed to another evacuation centre.
As she prepared to evacuate to a friend's home on higher ground, Waialua resident Kathleen Pahinui told The Associated Press in a phone interview that the aging dam is a concern every time it rains.
"Just pray for us," she said.
"We understand there's more rain coming."
Molly Pierce, spokesperson for the Honolulu Department of Emergency Management, said the evacuation order covers more than 4000 people, though the number could be higher.
Officials issued a warning for the dam during heavy rain last week, but the water level receded as rain subsided.
"The water is actively running over the spillway right now," she said.
The state regulates 132 dams across Hawaii, most of them built as part of irrigation systems for the sugar cane industry, according to a 2019 infrastructure report by the American Society of Civil Engineers.
In 2006, seven people were killed when the Ka Loko dam on the island of Kauai collapsed and water rushed downhill.
