"We thought we had pretty much one controlled and a new one broke out," Maui Mayor Michael Victorino told reporters at a news briefing.
He roughly estimated the flames scorched 19 square miles (50 square kilometers) of mostly former sugar cane land since alighting Thursday.
The new fire emerged at about 1:30 p.m. in fields near several retail stores on the edge of Kahului town. Maui police were evacuating customers from a Target store. A Lowe's store had already voluntarily evacuated.
Maui Fire Services Chief Rylan Yatsushiro said the fire was moving fast amid brisk winds.
Authorities issued mandatory evacuation orders for the historic sugar mill in Puunene and Puunene School, a former school built by Hawaiian Commercial and Sugar Co. in 1922.