Tropical Cyclone Cook has buffeted Vanuatu with strong winds, heavy rain and rough seas as it moves south-west away from the Pacific Island nation and towards New Caledonia.
The Vanuatu Meteorological and Geo-Hazards Department said the category two cyclone generated wind gusts of up to 130 kilometres per hour near its centre.
Authorities said the heavy rain and rough seas whipped up by Cyclone Cook would cause flash flooding in some parts of the country.
Nol Steven, from the country's National Disaster Management Office, said people in low-lying areas near rivers and along the coast would be most at risk.
"We have lots of rain and experiencing strong winds and looking at the ocean we have a big heavy swell," he said.
"We continue to inform those who live in low-lying areas to move to safer places and those who are in old buildings, they need to evacuate."
Cyclone Cook also brought down trees and cut power to some parts the capital, Port Vila, and Efate island.
Charlie Damon, from CARE International, said several hundred people left their homes as the cyclone intensified overnight.
"There were some families that did evacuate. Up to 350 people overnight did go to drier areas," he said.
"Our main concern will be those that have been displaced and making sure that any evacuation centres that they go to are safe, have enough clean water and that they're fed.
"And then after that will be the usual flood clean-up and any potential increase in water-borne disease."
Pictures posted on social media show fallen trees and swollen creeks but authorities did not report any major damage to buildings.
Port Vila resident Ben Bohane said Cook's rapid intensification from a tropical low caught many people by surprise.
"I think the first anyone really knew about the cyclone was Friday afternoon, so there wasn't much time to prepare."
"Where I am there's no power and no water," he said.
"I'm just on the edge of Port Vila but I'm told that in town power is still functioning and at least the phone lines are still working so that's good."
Cyclone Cook is tracking towards neighbouring New Caledonia, to the south west of Vanuatu.
Vanuatu Meteorological Services said the system remained slow moving and was continuing to build in intensity.
It is expected to reach category three by mid-morning on Monday and maintain that strength as it makes landfall over New Caledonia on Monday night or early Tuesday morning.