South Pacific islands face a drought set to create food shortages in the region, officials in Wellington warned after a second community declared a state of emergency due to lack of water.

Tokelau, a New Zealand-administered territory of about 1,400 people, had less than a week's drinking water after a long drought blamed on a La Nina weather pattern, Foreign Minister Murray McCully said on Tuesday.

McCully said Tokelau declared a state of emergency late Monday, following a similar move in neighbouring Tuvalu, where a New Zealand air force plane landed on Monday carrying containers of water and desalination units.

"There's been a state of emergency declared in Tokelau as well, where there are three islands, (they are) New Zealand citizens and they're down to less than a week's drinking water there too," he told Radio New Zealand.

McCully said other islands in the South Pacific were also reporting water shortages and New Zealand was carrying out a regional assessment amid fears the drought could lead to crop failures and food shortages.

"We're now doing an assessment, not just in Tuvalu but also in other areas of the Pacific that are affected by the shortage of rainfall, making sure we deal with the drinking water issue most urgently," he said.

"There are going to be some flow-on effects here, clearly this is having a severe impact on crops, so there's likely to be a food shortage as well."

He said the situation was urgent in parts of Tuvalu.

"There's less than a week's supply of drinking water on Funafuti, that's the main island in Tuvalu," he said.

"I understand one of the other outlying islands, Nukulaelae, has a more urgent shortage and there is a desalination plant on the way there."

McCully said New Zealand, a major aid donor in the Pacific "may yet be called upon to help in some other places". He did not specify where.

Tuvalu, one of the world's smallest independent nations with less than 11,000 residents, lies about halfway between Australia and Hawaii. Tokelau is is about 500 kilometres (310 miles) to the east.

A Red Cross situation report on Tuvalu released last week said the former British colony relied mostly on rainwater, which had been scarce this year because of the La Nina weather pattern.

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