Weather will likely play an important role when you plan your trip to Guadalcanal Island, but bear in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually April and May, with an average temperature of 27°C, while the coldest months are August and July, with an average of 26°C. The rainiest months in Guadalcanal Island are February, January, March and July, with each month seeing an average of 197 mm of rainfall.