New Zealand and Australia will Friday begin an airlift to help supply fresh water to the tiny drought-stricken Pacific nation of Tuvalu, which is under a state of emergency due to the crisis.

New Zealand Foreign Minister Murray McCully said a series of flights by Australian and New Zealand military transport planes would bring a large New Zealand Army desalination unit to the main island of Funafuti.

It follows the deployment of smaller desalination units there this week.

"The advice is that more capacity is needed to relieve the acute water shortage and replenish stocks," McCully said.

"At present the two operating desalination plants at Funafuti are producing a combined volume of 43,000 litres a day. The minimum requirement for the 5,300 residents is 79,500 litres a day."

Tuvalu is reliant on rainwater collection for drinking water and has been severely affected by a weather pattern across the Pacific known as La Nina.

The neighbouring New Zealand-administered territory of Tokelau has also declared a state of emergency after the drought left its 1,400 people with less than a week's supply of water.

Earlier this week McCully said the drought was not confined to Tokelau and Tuvalu, one of the world's smallest independent states with less than 11,000 people, and could cause food shortages across the South Pacific.

La Nina causes extreme weather, including both drought and floods, and was blamed for deluges and floods in Australia, Southeast Asia and South America late last year and earlier this year.