Melburnians are being warned not to swim at three key beaches due to high faecal bacteria levels.
The Environment Protection Authority is forecasting poor water quality at Mentone, Port Melbourne and Frankston today.
Swimmers are advised to avoid the water or risk contracting gastro.
Thunderstorms are also forecast for Melbourne today, meaning there is further risk of contamination in Port Phillip Bay.
12 other beaches are rated as fair, while 21 are rated as good. Check the Yarra & Bay beach report for the latest details.