Overflow Relief Gullies

The Overflow Relief Gully (ORG) is a critical part of your sewerage system.

Residents are encouraged to become familiar with the location of the ORG or ORGs at their residences.

They should be kept clear of obstructions such as pot plants and water should not be drained into them. Ensure the ORG is raised to the correct height and the grate is not fixed so it can pop off if the sewer backs up.

If the sewer backs up it will overflow through the ORG which is outside your house. If your ORG is blocked or covered over, the sewer backs up it and can overflow inside your house, often via a toilet.

Sewage in a yard is much easier to deal with than sewage through a house.

  • Overflow relief gullies are usually at the end of the house sewer system, close to the house.

    From the gully, the sewer line heads straight for the sewer main connection.

    On a large house with several bathrooms, there may be more than one ORG.

    The ORG is always set lower than the floors in your house that have floor wastes, such as the bathroom, toilet and laundry. This is so that if the sewerage system gets a blockage, it will overflow from the relief gully outside your house, not in your bathroom.

  • There are many reasons why the sewage could back up into a property.

    Tree roots invading the pipes inside a property before the mains connection is quite common, particularly at residences with old terracotta sewerage pipes which have cracked over time.

    The roots invade looking for moisture and these clog the pipe making it susceptible to blockages.

    Oils, fats or food wastes can also block a house sewer lines so always dispose of excess waste in the rubbish bin before dishwashing.

    If it does block, and your ORG is clear, then the overflow will occur outside, instead of inside, your house.

    A plumber should then be contacted to unblock the drain. Rectification work may also be required.

  • Overflows can occur when there is damage to the main sewer line downstream of your property or there is excessive stormwater entering the sewerage system.

    It is important to have correctly functioning ORGs so that the inside of your house does not get flooded in this event.

  • Directing stormwater into the sewer is prohibited under the Water Supply (Safety and Reliability) Act 2008.

    It is illegal to direct stormwater into the sewer either through plumbed drainpipes or by lifting manholes.

    Sewers are not designed for stormwater. Directing stormwater into them overloads the system and causes unnecessary sewer overflows which are expensive and hazardous events.

    ORGs are normally set slightly above ground-level to stop stormwater running into them and entering the sewerage system.

  • It is important to ensure the grate on your ORG is loose so that if it gets clogged or blocked, the pressure will lift it off and the gully will overflow outside.

    The image below shows the ORG at ground level and the grass clippings across the top indicate that rainwater drains into it. The ORG is not designed for this. 

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    Rainwater channelled into the sewer overloads the system and increases the likelihood of the sewer backing up. When a new home is built the ORG is installed at the correct height for its plumbing certificate. However, when landscaping is done, the ORG’s function as a sewer overflow can be compromised because the surrounding earth is made level with it or even sloped to drain into it.

    Residents should ensure ORGs are raised above the surrounding area and undertake rectification work if required.

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Sewage Treatment