The Fiji Meteorological Services says that TD05F has developed into a Tropical Cyclone and was named Irene at 6 pm yesterday.
It is a category one system with maximum winds near the centre of 65 km/hr with gusts up to 90 km/hr.
TC Irene is expected to move southeast between New Caledonia and southern Vanuatu.
The weather office says it is closely monitoring its progress and will keep Fijians updated.
Meanwhile, the FMS says an active trough of low pressure is expected to approach the Fiji group later today.
It says the depression will bring about heavy rain, and communities in flood-prone areas are advised to move to higher ground.