About heavy rainfall events

Heavy rainfall events increase water flow in waterways and may cause flooding.  

During high flows and flooding, contaminants such as animal poo or litter often enter waterways. This means there’s usually poor water quality after heavy rainfall events.

You shouldn’t swim in waterways for 48 hours after heavy rainfall events.

How heavy rainfall events impact the urban stormwater system

Urban stormwater systems aim to prevent flooding. Heavy rainfall events increase pressure on urban stormwater systems.

Heavy rainfall events can impact small and large construction sites. Overflow with sediment and other pollutants can run off into the stormwater system and waterways.

Find out more about how stormwater can become polluted.

How heavy rainfall events impact sewerage systems

Excess water can get into sewerage pipes during or after heavy rainfall events. This can cause increase sewerage system flows by up to ten times, pushing systems to their limit.

System overflows are designed to go into local waterways through Emergency Relief Structures (ERSs). An ERS protects sewerage systems from damage and prevents sewage overflow into houses, particularly during extreme rain events.

Local water corporations manage and maintain ERSs. These overflows cause discharges that can impact rivers and beaches.

Local water corporations let EPA know about overflows. We place water quality alerts on our website and Twitter account. You shouldn’t swim in these waterways for 48 hours after heavy rainfall events.

How heavy rainfall events impact Wastewater Treatment Plants

Heavy rainfall events can cause inflows too high for Wastewater Treatment Plants (WWTPS) to manage. This means the treatment process won’t work as it would normally.

High inflows can also fill WWTP treatment lagoons, resulting in spills to nearby waterways. WWTPs are licenced to discharge treated water of a certain quality into nearby waterways. When discharging water lower than the licenced quality, WWTPs may need to put up warning signs along waterways.

WWTPs are sometimes licenced to make emergency discharges. EPA advises no swimming near emergency discharges for 48 hours, or until the local water corporation says it’s safe.

How heavy rainfall causes blackwater events and fish deaths

Heavy rainfall events can wash:

  • large amounts of forest material, such as leaves and wood, into waterways
  • organic matter from industrial sites into waterways.

This can cause ‘black water’. It’s a natural process that can cause short-term harm, such as fish deaths. You can report fish deaths to EPA on 1300 372 842. We’re here 24 hours.


How La Nina events increases the chance of flooding

The La Niña weather pattern can cause above average rainfall across Victoria, especially in summer. La Niña events are part of the El Nino Southern Oscillation (ENSO), in which Pacific Ocean temperatures impact worldwide weather patterns. La Niña events increase the chance of severe flooding.

La Nina can also combine with the ‘negative Indian Ocean Dipole' (IOD) and increase the chance of above average rainfall in other seasons. The IOD is the difference in ocean temperatures between the west and east tropical Indian Ocean. Both IOD and ENSO can shift moisture towards or away from Australia. 

How to manage waste after a flood

About Victoria's water environments

Protecting Victoria's water

Monitoring Victoria's water


Alternative water supplies and their use


Recreational fishing and your health

How to report and respond to fish deaths

Prevent water pollution from your business

Current projects and issues

Reviewed 20 October 2022