Lisa...Scott is right. Stewart will set you straight. Be sure and send us your photos of the trip!
Sorry for tardiness, festivities got in the way of the PC (no bad thing!).
Kadavu has a typical tropical climate with the winter months being May to September and the summer being October to April. In winter time, Fiji experiences the South Easterly trade winds. These trades vary in strength from a light breeze to a 20kt blow and usually last about 5-6 days. Following a tradewind will be a couple of days of calm settled weather, usually with a light Northerly breeze before the next trade picks up.
NE trades usually mean breezy but sunny weather. Northerly winds mean calm seas but occasional showers as cloud over the hills is pushed over the coast.
The air temperature generally reaches a low of about 22 degrees in August / September though it can dip lower on rare occasions. Water temperature drops to a low of about 24 degrees.
If the REAL question is the unnameable subject of Cyclones....
The South Pacific can experience Cyclones when the water temperature exceeds 28 degrees. Cyclones therefore only occur in summer and at this latitude, tend to move fast, being through and gone within 24 hours. Cyclones in the South Pacific are far fewer in number than in the Northern hemisphere (hurricanes / Typhoones).
The Southern hemisphere averages less that a quarter of the number formed in the Northern hemisphere each year. The resort gets several days notice of the formation of a Cyclone anywhere in the South Pacific and any Cyclones that do form are monitored frequently.
So in a nutshell, don't be too concerned, Kadavu has great beaches, villages and coves for exploring in a Kayak, and if weather goes bad, you can be safe for the couple of days while it passes on the island. It is HIGHLY unlikely though!