Muddy floodwaters smothered vast stretches of Oahu's North Shore, a community world-renowned for its big-wave surfing.
Raging waters lifted homes and cars and prompted evacuation orders for 5500 people north of Honolulu, though they were later lifted. Authorities cautioned that a 120-year-old dam could fail.
On the island of Maui, authorities upgraded an evacuation advisory to a warning for some parts of Lahaina, which is still reeling from a deadly 2023 wildfire, because of retention basins nearing capacity.
North Shore Oahu residents who did not evacuate were heartened in the morning by receding waters and moments of blue skies, but more rain was on the way.
"Don't let your guard down just yet," said Tina Stall, a meteorologist with the National Weather Service in Honolulu, "there's still potential for more flooding impacts".
Governor Josh Green said the cost of the storm could top $US1 billion ($A1.4 billion), including damage to airports, schools, roads, homes and a Maui hospital in Kula.
"This is going to have a very serious consequence for us as a state," Green said at a news conference.
Green said the flooding was the state's most serious since 2004, when homes and a University of Hawaii library were swamped.
Dozens and perhaps hundreds of homes have been damaged, but officials have yet to fully assess the destruction.
Officials blamed some of the devastation on the sheer amount of rain that fell in a short amount of time on saturated land. Parts of Oahu received 20-to-30cm, the National Weather Service said.
More than 200 people were rescued from the rising waters, authorities said, but no deaths were reported and no one was unaccounted for. Crews searched by air and by water for stranded people.
Officials have been closely watching the Wahiawa dam, which has been vulnerable for decades, saying it was "at risk of imminent failure".
Water levels in the dam about 30km northwest of Honolulu, on the island of Oahu, receded by late Friday and then went up again with overnight rain.
However the dam appeared to be less of a concern the following morning than the "breadth of hazardous conditions" across the island, said Molly Pierce, a spokesperson for Oahu's Department of Emergency Management.
