If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Mount Cook: The hottest months are usually February and January, with an average temperature of 7°C, while the coldest months are July and August, with an average of -3°C. The rainiest months in Mount Cook are May, September, December and July, with each month seeing an average of 360 mm of rainfall.