The cyclone, whose strength had peaked last weekend when it crossed northern Tonga and Fiji's Lau group, is changing direction again.
After being on a northwesterly trajectory, Cyclone Ula is forecast to turn towards the southwest.
By Monday its centre is predicted to reach the southernmost islands of Vanuatu and weaken to a category one system.
A storm warning is in force for ships within 500 kilometres of Ula.