Why storms bring poo to city beaches

Several Melbourne beaches have been deemed as unsafe for swimming in the past week due to fecal bacteria in the water.

But this kind of problem isn't isolated to the Victorian capital, Environment Protection Authority Victoria's Anthony Boxshall tells AAP.

"While the attention has been on Melbourne, this could happen anywhere. It's just the reality of living in big cities".

HOW DOES FECAL BACTERIA END UP IN WATER?

When rain hits hard surfaces, it can pick up wastes and flush them into stormwater drains, creeks and rivers.

This includes dirt, detergents and animal poo.

This means water quality can decrease soon after heavy rain.

WHAT IS TESTED?

Environment authorities usually test for increased levels of enterococci, a group of bacteria found inside warm-blooded animals.

While enterococci won't make you sick, it is a good indicator water is contaminated with fecal bacteria.

WHAT CAN HELP INCREASE WATER QUALITY?

Stormwater gardens and plants help filter the water, while sunshine can help break down the bacteria in the bay.

"Nature can do better at filtering water than we can," Dr Boxshall says.

MELBOURNE'S POINT OF DIFFERENCE

The only way water can escape Port Phillip Bay is via evaporation or the small 3.5km ocean outlet in the Bay's south.

Dr Boxshall says studies have shown a packet of water takes almost a whole year to circulate out again.

"It makes it a little bit unusual and that's why we're so vigilant with the water quality," he says.

"Despite that, the water quality in Port Phillip is remarkably good for the fact it is the working front yard of a city of four million."

HOW TO STAY UP-TO-DATE WITH WATER QUALITY?

* Victoria - www.yarraandbay.vic.gov.au

* NSW - www.environment.nsw.gov.au/beach

* South Australia - www.epa.sa.gov.au

* Western Australia - health.wa.gov.au

* Northern Territory - nt.gov.au/emergency/community-safety/darwin-beaches-water-quality

* Tasmania, Queensland - check local government websites

(Sources: EPA Victoria, World Health Organisation)