Planning for the weather is a good way to make your trip to Guadalcanal Island relaxing and enjoyable, especially when you're splurging. 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.