A deadly and unexpected heatwave sweeping across Japan has killed 11 people – including a theme park mascot.

The 28-year-old was dancing in a 16kg outfit as part of his role as a fairy character at Hirakata Park, Osaka.

The part-time worker was rehearsing with other staff members when he fell ill and lost consciousness at around 7.30pm.

The temperature was reported to be about 28°C at that time.

Japanese locals keeping cool in the shade. Credit: AP

In a statement, the theme park said: "We would like to offer a deep apology from the bottom of our hearts.

"We will find the cause of it and work to prevent it from happening again.”

The park has cancelled all performances using character outfits during the heatwave.

The theme park worker's death is one of thousands of casualties in the heatwave sweeping across the nation.

Authorities have advised people to stay hydrated and take regular breaks. Credit: AP

More than 5,600 people have been hospitalised due to heat stroke symptoms, as temperatures soared across the country,.

People 65 and older accounted for more than 50% of casualties, according to the Fire and Disaster Management Agency.

Authorities have advised people to stay hydrated and take regular breaks, as it is predicted temperatures won't drop in the coming week.

However, this isn't the only time the country has suffered from a deadly heatwave.

Last year, a natural disaster was declared after 65 people died and over 22,000 were hospitalised from the heat.