The Fijian government and international aid agencies have begun delivering much needed aid to the Pacific nation's remote islands and coastal villages devastated by a powerful cyclone that killed 42 people.

The death toll has crept up in the days since Cyclone Winston struck Fiji late on Saturday as communication has gradually been restored with the outer reaches of the archipelago that is home to some 900,000 people.

Thousands of people are still sheltering in evacuation centres, their homes destroyed by winds or flooded by the most powerful storm to ever strike a Pacific nation.

An emergency response team from the International Red Cross reached the outer island of Koro, the seventh-largest of Fiji's 300 islands and one of the worst hit, by ship yesterday evening.       

"We don't know how many jetties are destroyed on the outer islands and whether ships carrying aid can land," said Dylan Quinnell, a spokesman for aid agency Care Australia in Fiji's capital Suva.   

Australia and New Zealand said they were ready to deploy helicopters to reach other areas that remained cut-off.   

"We have also sent two P3 Orion surveillance aircraft and they will be carrying out daily assessment of the damage particularly in those outlying islands," Australian Foreign Minister Julie Bishop told parliament.

The Fijian agriculture ministry told local media the cyclone has completely destroyed crops across the island, while charity Save The Children said scores of schools have been destroyed or badly damaged.

Thousands of Fijians live, learn and work in tin or wooden shacks in low-lying coastal areas.     

Aid workers have warned of potential outbreaks of the Zika and Dengue viruses, both carried by mosquitoes which could breed in the stagnant water left by the storm.