Their rainy season is Mid-November to mid-March. However, it's not like it's monsoons or anything - it's more like afternoon showers that pass quickly.
In late May, I stayed at Bora Bora Le Meridien pre-cruise, 4- days - it was fantastic! Then did the Radisson. Loved it - so much I'm going again next month!
Do a google search of Tahiti message boards to find more info - there are several.