Perth is set to swelter through a six-day heatwave, prompting a warning from the Health Department to residents, beachgoers, and cricket fans heading to the WACA.

The temperature is expected to hit 33 degrees today and the maximum will not be less than that until at least next week.

The Bureau of Meteorology is forecasting a maximum of 40 degrees on Monday, and defines a heatwave as at least three consecutive days over 35 degrees.

Thousands of people will pack the WACA ground on Friday for the start of the third Ashes Test.

Professor Tarun Weeramanthri from the WA Health Department says they need to take it easy.

"Drink enough water so you don't get dehydrated, be sun smart if you're outside and avoid or limit alcohol," he said.

"And it's that particular combination that's really important if you're thinking of being outdoors either watching sport or playing sport this coming weekend."

Professor Weeramanthri says others also need to be taking care with the elderly and the very young the most vulnerable to the heat.

"The advice is stay out the heat if you can," he said.