Tourists and locals alike braced for double trouble in Hawaii on Thursday as a rare pair of hurricanes took aim at the holiday paradise, the first due to make landfall within hours.

Big Island was expected to see a direct hit from Hurricane Iselle in the evening, bringing strong winds, heavy rains and dangerous storm surges, the Central Pacific Hurricane Center (CPHC) warned.

In an unusual development, Iselle was being trailed by another, stronger hurricane dubbed Julio, with the prospect of a one-two punch putting the popular archipelago on even higher alert.

State authorities shuttered all non-emergency public facilities and sent employees home from midday. Local schools were also being designated as shelters in the event of emergency evacuations.

Rain was already falling on the wind-turbine dotted northwest of Maui island, where locals say it almost never rains.

The island's 150,000 residents annually welcome 2.5 million tourists.

But even the public beaches were closing down, although sun seekers jumped the waves, ignoring red flag warnings of hidden riptides.

Maui authorities have banned dive and snorkel boats from setting out to popular areas, including the unspoiled half-sunk Molokini crater famed for its undersea coral.

Some hotels were even closing their pristine pools, and disappointed guests found they could not rent boogie boards or paddleboards to brave the swelling waters of the Pacific Ocean.

"In 22 years here, I've never seen anything of this magnitude," store owner Tim White told AFP. "Yesterday, some people were even scrambling to get out early ahead of the storms."

But he remained unconcerned about the approaching storm.

"We have had so many false alarms, including about tsunamis, I'm not too worried," White said.

- Storm approaching -

Packing maximum sustained winds of 85 miles (140 kilometers) per hour, Iselle -- a Category 1 storm -- was 255 miles east-southeast of Hilo and 460 miles east-southeast of the state capital Honolulu at 1800 GMT, CPHC forecasters said.

"Remember that the effects of hurricanes are far reaching. Do not focus on the center position alone," they warned.

Tropical storm conditions were expected on Big Island on Thursday afternoon, with hurricane conditions taking hold overnight. Maui and Oahu were forecast to see tropical storm conditions starting late Thursday.

The haven for sun worshippers and water sports fanatics from around the world was expected to see rainfall of up to 12 inches (30 centimeters) thanks to Iselle.

"These rains could cause life-threatening flash floods, as well as rock and mud slides," the CPHC cautioned.

Julio, which strengthened to a Category 2 storm overnight, was situated some 1,155 miles east of Hilo, according to the latest update from the National Hurricane Center.

With maximum sustained winds of 105 miles per hour, NHC forecasters predicted it would lose steam over the next two days.

On its current trajectory, Julio was expected to pass to the north of Big Island as a tropical storm late Saturday or Sunday.

With the twin storms fast approaching, television images showed long lines at local supermarkets, as residents and vacationers alike rushed to stock up on water and other basics to see them through the next few days.

The Honolulu Star-Advertiser reported that shelters would open for residents of Oahu -- home to Honolulu -- and that state authorities were shutting down recreation areas that could become danger zones due to flash flooding and other storm-related hazards.