Hong Kong on Sunday experienced its third hottest day since records began in 1884.

The Observatory said the thermometer reading at its headquarters went as high as 36.1 degrees Celsius in the afternoon, making it the hottest day ever for July.

It also noted that Sheung Shui saw its mercury level shooting up to 39 degrees, and people in most places in Hong Kong sweltered in temperatures over 35 degrees.

Meanwhile, a 60-year-old man died after falling unconscious during a hike at Lantau island's West Dog's Teeth.

Police said they received a report at around 10am that the man was unconscious at the site.

He was declared dead at the scene.

Investigators said the cause of death will have to be confirmed by autopsy.

It follows the death on Saturday of a man during a hike at Sharp's Peak in Sai Kung. He's thought to have suffered heatstroke and was declared dead after being airlifted to hospital.

The sizzling heat is expected to continue for the rest of the month, with temperatures forecast to stay around 36 degrees on Monday.

The Observatory reminded the public to avoid staying under the sun for too long, stay hydrated and beware of heatstroke.