"It's most likely going to be one of the severest cyclones to hit the country in recent times," he told The Associated Press.
On its present track the cyclone will pass over the Lau and Lomaiviti island groups, home to 10,000 people, and across the Labasa area of Vanua Levu where 80,000 people live, he said.
Matt Boterhoven, the senior meteorological forecaster, warned that wind gusts of up to 250kph and 250mm of rain are going to "take out a lot of the small island groups".
People around the towns of Labasa, Savusavu and Nabouwalu on Vanua Levu were at risk, as were a number of tourist resorts, he added.
"We hold very serious concerns for the safety of the people there."
In 2003 major flooding brought by the lower-intensity Cyclone Ami killed 18 people.
Tropical storms are common in the South Pacific from November to April and range from category 1 to category 5.
The most powerful can pack sustained winds of 210kph.