The US Coastguard will ship drinking water to a remote New Zealand territory in the Pacific which faces a state of emergency due to severe drought, Wellington said Wednesday.

The Coastguard ship "Walnut" will depart American Samoa on Thursday for Tokelau, a New Zealand-administered territory of about 1,400 people which has less than a week's supply of water left, Foreign Minister Murray McCully said.

Tokelau was reliant on collecting rainwater for drinking and had been severely affected by the La Nina weather pattern, he said.

Both Tokelau and neighbouring Tuvalu have declared states of emergency as their water supplies dwindle.

McCully said a New Zealand air force plane would arrive in American Samoa on Wednesday carrying relief personnel and empty water storage units that would be shipped to Tokelau on the Walnut.

He said the US vessel had also used its on-board desalination plant to produce about 136,000 litres (30,000 gallons) of drinking water.

"The Coastguard vessel will travel to Tokelaus three main atolls. New Zealand estimates that this, plus current stocks, will be a sufficient supply in the short term," McCully said.

"We thank our American friends for their willingness to assist and the speed at which the Walnut is able to be deployed."

McCully said New Zealand was also continuing relief efforts in Tuvalu, one of the world's smallest independent states with less than 11,000 people.

He said an air force plane had arrived with containers of water and desalination units on the main island of Funafuti earlier this week. Residents there have been rationed to 20 litres a day.

"Repairs are under way to the main desalination plant on Funafuti, and a desalination plant and Red Cross personnel arrived on the worst-affected island of Nukulaelae this morning (Wednesday)," he said.

McCully has warned the drought is not confined to Tokelau and Tuvalu and could cause food shortages across the South Pacific.

The La Nina weather pattern causes extreme weather, including both drought and floods, and was blamed for deluges and floods in Australia, Southeast Asia and South America over late 2010 and early 2011.

Tuvalu lies about halfway between Australia and Hawaii. Tokelau is is about 500 kilometres (310 miles) to the east.