A Defence Force Hercules carrying water containers would fly to Pago Pago in American Samoa tomorrow morning, where it would meet up with a US Coastguard vessel.
The ship, which has an onboard desalination plant to fill the containers, will then travel to Tokelau's three main islands.
Mr McCully said the water tanks would provide about 136,000 litres of water, which would be a sufficient supply in the short term.
New Zealand was continuing to provide help in Tuvalu, with work under way to repair the main desalination plant in the capital, Funafuti.
Local journalist Sila Lalua yesterday said one of the country's islands, Nukulaelae, was down to just 60 litres of water, and basic food had also run out.
"On all the other islands, water is short and island Kaupule [local government] are rationing out water at two 20 litre buckets of water per day per family," Ms Lalua said.
"There was little rain over the weekend on most of the islands but not enough obviously. The grass is still brown in colour and banana and breadfruit trees have seen better days."