SYDNEY, Australia (CNN) -- The residents of the Cook Islands in the South Pacific are bracing for their third major storm in two weeks as "super cyclone" Olaf heads southeast towards the sparsely populated nation.

The Category-5 storm -- the most powerful in weather forecasting terms -- has skirted major population centers in Samoa and American Samoa but has made a direct hit on the latter's Manu'a Islands.

The Australian-Pacific Center for Emergency and Disaster Information (APCEDI) reports that the eye of Olaf passed over the three Manu'a Islands bringing heavy rains and flooding but no reports of casualties.

"The 1,300 residents of the 3 Manu'a Islands, Ta'u, Ofu and Olosega, have weathered the storm in shelters on each island," the APCEDI's Web site reports.

However, a New Zealand Air Force plane has been dispatched to the area north of Samoa to search for three fishing vessels with 18 people aboard believed to have been caught in the storm, according to wire reports.

One of the boats is known to have sunk about 150 kilometers (90 miles) north of Samoa and there are fears for the safety of the crew.

Cyclone Olaf is packing sustained winds of 255 km/h (155 mph) with gusts of over 300 km/h (185 mph) and causing 10 meter (30 foot) waves in coastal areas.

A state of emergency was earlier declared in Samoa and American Samoa and schools, businesses and airports closed.

The storm is now heading directly for the southern Cook Islands, following hard on the heels of cyclones Meena and Nancy.

The tiny nation of Niue has also been placed on alert as it may also receive a glancing blow from the storm, the APCEDI reports.

"This continues to be a critically dangerous situation for the Manu'a group in American Samoa for the next four to six hours until Olaf pulls away," the center says.

"The Cook Islands and Niue must now be very vigilant and monitor this storm very closely."

The 15 islands of the Cook group have a total land area of 230 sq km (90 sq miles). From north to south, the islands are spread over 1,400 km (900 miles).

The island's total area of jurisdiction covers 2,200,000 square km (850,000 sq miles) of ocean.

The Cooks economy is heavily reliant on tourism revenue generated from its pristine atolls and blue-water lagoons.