A returned overseas traveller has tested positive for measles in WA, prompting a public health alert for three busy Perth locations.

WA Health issued the alert on Wednesday, warning people who visited the exposure sites at the below specified times to monitor for symptoms of the highly contagious virus.

The exposure sites include Mirrabooka Square — including Woolworths, Mirrabooka Fresh Fruit and Shaheen Store — Taylor Road IGA in Nedlands and the Joondalup Health Campus Emergency Department.

People who have been exposed should monitor symptoms for 7 to 18 days after their visit and seek medical advice if symptoms develop.

Early symptoms include fever, tiredness, cough, a runny nose and sore eyes. A red, non-itchy rash typically also develops three or four days later.

Anyone who develops symptoms is advised to put on a mask, isolate and seek medical advice.

It is further advised that they call their GP or hospital emergency department before attending so staff can take precautions to protect other patients and healthcare workers.

People born after 1965 who have not received two doses of a measles-containing vaccine are considered not immune.

Young children, pregnant women and people with weakened immune systems are among those at risk of severe illness.

For more information visit public health alert online.

New exposure sites:

• Mirrabooka Square: Tuesday October 20, between 5.50pm to 6.40pm
• Taylor Road IGA 24 hours, Nedlands: Tuesday October 20 between 9.00pm to 9.45pm
• Joondalup Health Campus Emergency Department: Monday October 26 between 12.30pm to 1.30pm