High mosquito numbers are expected in WA as the weather warms up, with people urged to be alert and take precautions to avoid being bitten.

There will be more mosquitoes and a greater risk of mosquito-born diseases in Western Australia as the weather warms up, the Health Department warns.

People living in Perth and WA's South West region are urged to be alert and take precautions to avoid being bitten over spring and summer.

Michael Lindsay, from the Health Department, said while many parts of the state had experienced a below average number of mosquitoes last summer, higher numbers were expected to return this year.

"Recent substantial rainfall combined with increasing temperatures and predictions of higher than usual tidal activity in coming months are likely to favour increased breeding of mosquitoes," he said.

Mosquitoes in Perth and the South West can transmit Ross River virus and Barmah Forest virus.

"There is no vaccine or specific cure for either disease. The best way to avoid infection is to prevent mosquito bites," Dr Lindsay said.

Symptoms for both viruses can last for weeks and include painful or swollen joints, sore muscles, skin rashes, fever, fatigue and headaches.

The department advises people to visit their GP and have a blood test if they experience symptoms.

To avoid being bitten, avoid outdoor activities around dawn, apply a repellent, empty or cover standing water to reduce mosquito breeding, and use mosquito nets when camping.