* New Zealand navy ships to arrive in Tonga on Friday
* Airport could re-open on Thursday
* Australian PM speaks to Tongan counterpart
* U.N. planning remote relief operations
* (Adds Australian leader's comment; paragraphs 14,15)
By Praveen Menon, Kirsty Needham and Tom Westbrook
Jan 19 (Reuters) - Two New Zealand navy vessels will arrive in Tonga on Friday carrying critical water supplies for the Pacific island nation reeling from a volcanic eruption and tsunami and largely cut off from the outside world.
Hundreds of homes in Tonga's smaller outer islands have been destroyed, with at least three deaths after Saturday's huge eruption triggered tsunami waves that rolled over the islands, causing what the government calls an unprecedented disaster.
With the nation's airport smothered by volcanic ash and communications badly hampered https://www.reuters.com/markets/funds/undersea-cable-fault-could-cut-off-tonga-rest-world-weeks-2022-01-18 by the severing of an undersea cable, information on the scale of devastation has come mostly from reconnaissance aircraft.
The Red Cross said its teams in Tonga had confirmed that salt water from the tsunami and volcanic ash were polluting the drinking water sources of tens of thousands of people.
"Securing access to safe drinking water is a critical immediate priority ... as there is a mounting risk of diseases, such as cholera and diarrhoea," Katie Greenwood, a Pacific official of the International Federation of Red Cross and Red Crescent Societies, said in a statement.
The Hunga Tonga-Hunga Ha'apai volcano erupted with a blast heard 2,300 km (1,430 miles) away in New Zealand and sent tsunami waves across the Pacific Ocean.
James Garvin, chief scientist at NASA's Goddard Space Flight Center, said the force of the eruption was estimated to be the equivalent of five to 10 megatons of TNT, or more than 500 times that of the nuclear bomb the United States dropped on the Japanese city of Hiroshima at the end of World War Two.
New Zealand's foreign ministry said Tonga had approved the arrival of its ships, the Aotearoa and the Wellington, in the COVID-free nation, where concerns about a potential coronavirus outbreak are likely to complicate relief efforts.
Simon Griffiths, captain of the Aotearoa, said his ship was carrying 250,000 litres of water, and had the capacity to produce another 70,000 litres a day, along with other supplies.
"For the people of Tonga, we're heading their way now with a whole lot of water," Griffiths said in a statement.
The Polynesian archipelago of 176 islands, 36 of them inhabited, has a population of about 105,000. Its Fua'amotu International Airport was not damaged by the tsunami but it was covered in ash, which has had to be cleared manually.
Aid flights from New Zealand and Australia could begin on Thursday, a Tongan official said, depending on the clear-up.
Australian Prime Minister Scott Morrison said two Hercules aircraft were ready to go with humanitarian supplies and telecommunications equipment "as soon as conditions allow".
"HMAS Adelaide is also preparing to depart from Brisbane with water purification equipment and additional humanitarian supplies," Morrison said on Facebook after he spoke with Tonga's prime minister, Siaosi Sovaleni.
As well as emergency supplies, Australia and New Zealand have promised immediate financial assistance. The U.S. Agency for International Development approved $100,000 in immediate assistance.
The Asian Development Bank was discussing with Tonga whether it would declare a state of emergency to draw on a $10-million disaster funding facility, senior bank official Emma Veve told Reuters.
Other countries and agencies, including the United Nations, are drawing up plans to help. China will send help, including water and food, when the airport opened, a spokesperson of its foreign ministry said.
Waves reaching up to 15 metres (49 feet) hit the outer Ha'apia island group, destroying all the houses on the island of Mango, as well as the west coast of Tonga's main island, Tongatapu, the prime minister's office said.
Residents of Tongatapu were being moved to evacuation centres as 56 houses were destroyed or seriously damaged.
New Zealand said power had been restored and Tongan authorities were distributing relief supplies.
But the country is largely offline since the volcano damaged the sole undersea fibre-optic communication cable and it would probably take a month https://www.reuters.com/world/asia-pacific/tonga-likely-spend-month-without-internet-cable-2022-01-19 or more to fix, its owner said.
A specialist ship should embark from Port Moresby on a repair voyage on the weekend, said Samiuela Fonua, chairman of cable owner Tonga Cable Ltd, but with up to nine days sailing to collect equipment in Samoa, he said it would be "lucky" if the job was done in a month.
International mobile phone network provider Digicel has established a 2G connection using a satellite dish, the New Zealand foreign ministry said, but it is patchy and amounts to about 10% of usual capacity.
Tongan communities abroad have posted images from families on Facebook, giving a glimpse of the devastation, with homes reduced to rubble, fallen trees, cracked roads and sidewalks and everything coated in grey ash.
The United Nations and aid agencies were preparing relief flights to Tonga but without personnel who disembark, so to avoid introducing the coronavirus, said Fiji-based U.N. co-ordinator Jonathan Veitch.
Tonga is one of the few countries free of COVID-19 and an outbreak there would disastrous, he added.
(Reporting by Praveen Menon, Kirsty Needham, Tom Westbrook, Karen Lema and Jane Wardell; writing by Robert Birsel; editing by Grant McCool and Richard Pullin)