A category five cyclone can have sustained winds of 252km/h.
The Vanuatu National Disaster Management Office said the provinces of Torba, Sanma, Penama and Malampa are now on Red Alert. The province of Shefa is now on Yellow Alert.
WeatherWatch.co.nz said the situation with Harold had "become much more serious overnight with the storm already exceeding initial Government forecasts".
It said the situation facing Vanutau was "very serious": "Secure shelter immediately."
"Earlier this week WeatherWatch.co.nz said that Cyclone Harold could be the most serious cyclone of the South West Pacific cyclone season due to the fact it could become severe and make landfall directly in a populated place. All other cyclones this season have been almost entirely at sea and weaker.
"Some positive news is that Port Vila now looks less exposed to a direct hit but several other islands to the north are now directly exposed."
Weatherwatch also sounded caution that the south-east corner of Fiji might also be in the path, although a direct hit remained unlikely.
A State of Emergency was declared by Vanuatu's President on 26 March in response to the Covid-19 pandemic.
The approach of Tropical Cyclone Harold may see a change to the government's state of emergency as people start to prepare for the category 4 cyclone.
In 2015, Tropical Cyclone Pam, a category 5 storm, devastated the Vanuatu group.
-RNZ
