If you're hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Koumac: The hottest months are usually February and January with an average temp of 79°F, while the coldest months are August and July with an average of 71°F. The rainiest months in Koumac are March, January, February, and April, with each month seeing an average of 7 inches of rainfall.