ABC A boat lies among trees stripped by Cyclone Ian on Tonga's Ha'apai islands.

In Tonga, the Ministry of Health is working with churches on the Ha'apai group of islands to provide counselling services for victims of Severe Tropical Cyclone Ian.

Director of Health, Dr Siale Akauola, told Pacific Beat that mental health services are a priority along with providing ample supplies of safe drinking water.

Four days after the category five cyclone devastated the Ha'apai group of islands, killing one person and making thousands homeless, the United Nations Office for the Coordination of Humanitarian Affairs has published an assessment of the damage.

It says more than half of the 1,130 buildings that were affected by Cyclone Ian have been destroyed and 34 percent of the rest have major damage.

It says that of the 17 primary and secondary schools affected in Ha'apai, 13 have major damage.

"Fortunately most of the people who've been evacuated to evacuating centres are living in large church buildings which still have adequate sanitation facilities so I'd say close to 90 percent of people are living in such church facilities," Dr Akauola said.

"So the question is sustaining the clean water supply and what happens when they move back into their damaged homes because almost 80 percent of dwellings or houses were either destroyed or have sustained major damage."

Dr Akauola says staff are taking action to ensure diseases like dengue fever or typhoid do not take hold by spraying for flies and mosquitoes as well as sourcing mosquito nets.

The United Nations situation report says the most immediate challenge is logistics to distribute relief and recovery items to outer islands as well as within affected areas.

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Relief distribution still a challenge on Tonga's islands