Fiji's MetService is warning it could even become a category four by midnight.
People in the province of Torba, Penama and Sanma will be directly affected in the next 48 to 72 hours by hurricane force winds of up to 185km/h.
Heavy rainfalls with flash flooding are expected over low-lying areas and areas close to river banks, including coastal flooding, over the Vanuatu group on Monday and Tuesday.
Very rough seas with heavy swells are expected over all Vanuatu waters. Seagoing vessels are advised to take extra precautions.
New Zealand MetService duty meteorologist Philippa Murdoch said the storm was a little ahead of the tropical cyclone season for the area.
"This one is slightly early," she said.
"The season usually starts at the beginning of November, but we've got all the right ingredients."
Murdoch said these factors included warm sea surface temperatures and quite light winds aloft, which means there was nothing to break it up.
The National Institute of Water and Atmospheric Research (NIWA) in New Zealand expects nine to 14 cyclones this season - nine being the long-term average.
Four to eight of these are expected to be severe.
NIWA meteorologist Ben Noll said El Niño had increased the risk.
"El Niño, it does tend to enhance the risk for cyclone activity near and east of the International Dateline, so for countries like Fiji, the Cook Islands, and many in between."
During El Niño 1982/83, there were 10 severe cyclones in the South Pacific.