The Category 1 cyclone is forecast to move towards Fiji at about 7km/h.
It is expected to move west, past Tonga's northernmost islands.
RNZI reports the new storm in the Pacific comes as Cyclone Donna, now Category 3, arrives in New Caledonia, with wind gusts up to 230km/h.
The Fiji Met Service said Ella was intensifying and was forecast to be Category 2 by Thursday.
It was expected to make landfall on Vanua Levu in Fiji late on Thursday evening or early Friday morning.