Backflow Prevention

What Backflow Means

Water systems are usually designed to move water in one direction. Backflow occurs when pressure, surcharge, blockage, flooding or system conditions cause water to move backwards through a connected system.

During heavy rain or flooding, external systems may become overloaded. If water cannot discharge normally, it may rise within pipes, pits, drains or connected fixtures and create a backflow pathway into lower parts of a property.

Backflow Can Be Different From Surface Flooding

Surface flooding is usually visible as water moving across the ground toward an opening. Backflow may appear from a pit, drain, fixture or connected system even when the surface entry point is less obvious.

• Water may rise from floor drains or pits
• Water may surcharge through stormwater connections
• Water may affect basement or low-level plumbing fixtures
• Water may enter from connected systems rather than external openings

This is why backflow risk should be considered separately from doors, ramps, driveways and surface drainage.

For a broader starting point on flood pathways and exposure, see the Flood Risk Guide. For practical mitigation categories, see the Flood Mitigation Guide.

Where Backflow Risk Can Occur

Backflow risk is often linked to low points, below-ground areas, connected drainage systems and fixtures that sit below external surcharge levels.

Properties with basements, below-ground car parks, low courtyards, drainage pits, pump-out systems or low-level amenities may need closer review by suitable contractors or professionals.

Common Backflow Risk Areas

• Basement car parks
• Below-ground plant rooms
• Low-level bathrooms, laundries or amenities
• Floor wastes and internal drains
• Sump pits and pump-out systems
• Stormwater pits and pipe connections
• Low courtyards and enclosed paved areas
• Commercial kitchens, washdown areas or service areas
• Loading docks and below-ground service yards

Practical Review Questions

• Are any drains or fixtures below surrounding ground level?
• Are there floor drains in basement or low-level areas?
• Can stormwater surcharge affect the property?
• Can sewer or wastewater surcharge affect low-level fixtures?
• Have backflow risks been reviewed after previous flooding or near misses?

Backflow risk is site-specific and should be reviewed by suitably qualified people where technical assessment is needed.

For below-ground areas, see Basement Car Park Flood Risk. For commercial site service areas, see the Commercial Flood Barrier Guide.

Stormwater, Sewer And Drainage Connections

Backflow prevention planning should consider which systems connect to the property and how those systems behave during heavy rain or surcharge conditions.

Stormwater, sewer and drainage systems may have different risks, different devices and different inspection requirements. They should not be assumed to operate in the same way.

Connection Information To Identify

• Stormwater connection points
• Sewer or wastewater connection points
• Internal floor waste and drain locations
• Sump pit and pump-out connections
• Downpipe and roof drainage connections
• Pits, inspection openings and access points
• Known overflow or surcharge locations

Questions For Contractors Or Professionals

• Which systems could surcharge during heavy rain?
• Are any low-level fixtures vulnerable to reverse flow?
• Are backflow prevention devices installed?
• Are existing devices suitable for the system they protect?
• Are inspection, testing or maintenance records available?
• Are there any compliance or access requirements for the devices?

These questions should be directed to licensed or suitably qualified people where plumbing, drainage or compliance matters are involved.

For related drainage and pump system checks, see Drainage Management and Pumps And Sump Systems.

Backflow Prevention Devices

Backflow prevention devices are intended to reduce reverse flow through selected systems or connections. The correct device, location and inspection process depends on the type of system and the risk being managed.

Property owners and managers do not need to diagnose the system themselves, but they should know whether backflow prevention devices are present, where records are stored and who is responsible for inspection or testing.

Device Information To Record

• Device location
• System or connection protected
• Installer or contractor details
• Installation date, where available
• Inspection or testing requirements
• Service reports or test records
• Access requirements for future inspection
• Any known defects, repairs or recommendations

Access And Identification

Devices should be identifiable to the people responsible for maintenance, but sensitive access information should be handled securely. Records should explain where device information is stored without exposing restricted access details publicly.

For maintenance planning and responsibility, see Regular Maintenance.

Warning Signs And Review Triggers

Backflow issues may not always be obvious until heavy rain, surcharge or a fault occurs. Warning signs should be recorded and reviewed by suitable contractors or professionals.

Warning Signs To Record

• Water rising from floor wastes or low-level drains
• Repeated odour, gurgling or slow drainage during rain
• Water marks near floor drains, pits or fixtures
• Stormwater pits surcharging during heavy rain
• Basement or low-level areas affected without obvious surface entry
• Previous incidents of reverse flow or surcharge
• Missing or outdated inspection records
• Devices that are inaccessible or not clearly identified

Review Triggers

• After a flood event, heavy rain event or near miss
• After drainage, plumbing or pump works
• After building works that alter levels, fixtures or drainage connections
• When inspection records are missing or outdated
• When repeated odour, surcharge or drainage issues are reported
• When responsible people or contractors change

Backflow warning signs should be recorded and reviewed promptly, especially in basements, plant rooms, commercial kitchens or low-level amenities.

For action planning across multiple resilience areas, see Flood Resilience Checklists.

Inspection, Testing And Records

Backflow prevention records help show what devices are installed, where they are located, when they were inspected and whether any defects or recommendations were identified.

The inspection or testing process depends on the system and the device. Property owners and managers should rely on licensed or suitably qualified people for technical assessment and servicing requirements.

Records To Keep

• Device location and identification
• System or connection protected
• Inspection or test date
• Contractor or licensed professional details
• Inspection or test result
• Defects or recommendations
• Repairs or actions completed
• Next inspection, test or review date

Store Records With The Flood Risk File

Backflow prevention records should be stored with the property’s broader flood risk documentation so they can be found during maintenance reviews, insurer discussions, contractor handovers or post-event investigations.

For a full documentation structure, see the Flood Risk Documentation Checklist. For insurance-related records, see Flood Mitigation And Insurance Resilience.

Backflow Prevention Checklist

Use this checklist as a starting point when reviewing backflow prevention information for a property.

Summary

Backflow prevention is an important part of flood resilience because water may enter through connected systems rather than obvious surface openings.

Key areas to review include low-level drains, stormwater connections, sewer or wastewater connections, sump systems, backflow prevention devices, warning signs and inspection records.

Backflow risks and devices should be reviewed by licensed or suitably qualified people where technical assessment, installation, testing or compliance advice is required.