The Italian government has issued extreme weather warnings for 16 cities, with temperatures expected to soar as high as 49 degrees.

Amidst record-breaking heatwaves, wildfires and evacuations, the government travel advisory service is encouraging Aussies to exercise caution on their European summer getaways.

Splendour in the Grass returned to the Byron Bay hinterlands for the first time in three years, however festival-goers were met with intense rain, mud and over 10 hours of queuing for some.

Multiple states are preparing for an extreme heat wave as temperatures exceed 40C, bringing one of the hottest December days in years.

A severe weather warning has been issued for eastern NSW with intense and heavy rain expected to cause more inundation in already saturated and flooded areas.