Dual Septic and Greywater Systems on Rural NSW Properties

A dual wastewater system separates the property’s waste streams into two parallel systems: a conventional septic (or AWTS) handling blackwater from toilets and kitchen, and a separate greywater treatment system handling shower, bath, basin, and laundry water. Together they manage the same total waste load as a single system, but with the potential for significant water reuse, reduced load on the septic, and better resource efficiency on rural properties.

This arrangement is increasingly popular with environmentally focused owners of lifestyle blocks in the Comboyne, King Creek, and Hastings hinterland areas where water conservation, soil health, and responsible land management are priorities.

Why Run Two Separate Systems?

The case for a dual system rests on two main benefits:

1. Greywater reuse for irrigation Greywater from showers and laundry is relatively low in pathogens compared to blackwater (it has no faecal content unless directly contaminated). When treated appropriately, it is a valuable irrigation resource, particularly during the dry months on the Mid North Coast when rainwater tanks are under pressure. A dual system allows this resource to be recovered and applied to the garden, lawn, or non-food vegetation.

2. Reduced load on the septic system In a typical household, 50-70% of total wastewater is greywater. Diverting this away from the septic reduces the hydraulic load on the tank and land application area by roughly half. This extends the time between pump-outs, extends the life of the absorption area, and allows a smaller or less intensive septic system to handle the remaining blackwater.

What Counts as Greywater?

In a dual system, the property’s internal plumbing is configured to separate:

To the greywater system:

• Shower and bath drains
• Bathroom basin drains
• Laundry (washing machine outlet, laundry tub)

To the septic system:

• All toilet waste (blackwater, always to septic)
• Kitchen sink (treated as blackwater in NSW due to food waste, fats, and detergents)
• Dishwasher

Note: kitchen sink water goes to the septic even in a dual system, it has too high an organic load to be treated as simple greywater in NSW’s regulatory framework.

How the Greywater System Works

A greywater treatment system in an approved dual-system installation in NSW typically consists of:

1. Greywater collection: A small collection tank receives greywater from the nominated fixtures. This is not a long-term storage tank, it typically holds water for only hours before treatment.

2. Treatment stage: Most approved residential greywater systems use biological filtration (biofilter or textile filter) to reduce organic matter, suspended solids, and some pathogens.

3. Disinfection: UV light or chlorination to achieve safe effluent quality for land application.

4. Land application: The treated greywater is applied to a defined irrigation area via subsurface drip lines or in some configurations surface spray on designated zones.

Systems must be approved under NSW Health’s greywater system approvals, only listed systems are acceptable to Port Macquarie Hastings Council for a formal dual-system DA.

The Approval Pathway for a Dual System

A dual system is not something you can install by connecting a garden hose to the shower drain. It requires full Council approval through a DA:

1. Pre-DA consultation: Contact Port Macquarie Hastings Council’s development and environment team to confirm they will accept a dual-system DA for your specific property and confirm what documentation is required.

2. Site assessment: Both the septic component and the greywater component need to be assessed for site suitability, separate land application areas, separate setback compliance, separate soil assessment in some cases.

3. System selection: Both system components must be approved models under their relevant Australian Standards and NSW Health listings.

4. DA lodgement: Combined DA covering both systems, with site plans, calculations, and specifications for each.

5. Installation: All work by licensed plumber and drainer, with inspection stages.

6. Registration: Both systems registered on the Council OSM register on completion.

See our Council septic approval guide for the DA process in detail.

Land Requirements for a Dual System

One of the practical constraints of a dual system is that you need two separate land application areas, one for the greywater system and one for the septic system. On a property with plenty of available land (10+ ha), this is straightforward. On smaller rural-residential lots (2-5 ha with setback constraints), fitting two separate treatment and dispersal areas may be challenging.

If your block is constrained, a AWTS single system that handles all wastewater is often more practical than trying to fit two separate systems on a tight lot.

Cost Comparison: Dual System vs Single AWTS

Dual systems cost more upfront because you are installing two complete systems. The payback comes through water savings (reused greywater reduces mains or tank water consumption) and reduced septic maintenance frequency (pump-outs needed less often with lower hydraulic loading).

For off-grid or water-scarce rural properties, the payback calculation can be compelling. For properties on mains water with adequate rain catchment, the financial case is weaker.

Maintenance for a Dual System

Owning a dual system means maintaining both components:

Greywater system:

• Six-monthly to annual service of the greywater treatment unit
• Annual check of drip irrigation lines
• UV lamp replacement annually
• Small collection tank pump-out every 3-5 years (some systems)

Septic system (blackwater only):

• Pump-out less frequently than a full-load system, every 5-7 years for a household with blackwater only
• AWTS servicing (if AWTS is used for blackwater), quarterly

What to tell your plumber/service provider: Make sure they understand the system configuration, particularly that the kitchen sink goes to the septic, not the greywater system. Misrouted flows are a common source of dual-system problems.

Suitable Properties for a Dual System

A dual system makes the most sense when:

• You are on a rural property with adequate land for two dispersal areas
• You are not connected to mains water and want to conserve tank/dam water
• You have a strong interest in sustainable waste management
• You are building or undertaking a major renovation that allows separation of plumbing at the design stage
• Your site has sufficient soil and area to support both systems at required setbacks

It makes less sense when:

• Your lot is small (< 2 ha) and setbacks are constrained
• You are retrofitting an existing built house (retrofitting dual plumbing is expensive)
• Your primary driver is just getting Council compliance and the simplest possible system
• You don’t want to manage two separate service requirements

FAQ

Is a dual greywater/septic system common in the Comboyne area? It is becoming more common on new builds and major renovations on lifestyle blocks in the hinterland area, but it remains less common than single-system installations. Most rural Comboyne properties with older systems run a single conventional septic that handles all waste.

Can I add a greywater system to an existing single septic property? Yes, subject to Council approval and sufficient land area. A licensed plumber would need to separate the internal plumbing (divert shower/laundry drains away from the main drain to the septic), and a new greywater system and its land application area would need full approval. This is a significant plumbing project in a built house.

Does the greywater system need quarterly servicing like an AWTS? Most greywater treatment systems have a less intensive service schedule than AWTS, typically 6-monthly. But this varies by the specific system approved. Check the system’s Council approval conditions.

What happens if the greywater system fails while the septic is running normally? If the greywater treatment system fails, you can temporarily redirect all wastewater to the septic system while the greywater system is repaired, provided the septic has capacity to handle the extra load. This is why having a diverter valve at the entry to the greywater system is recommended.

My neighbour runs their laundry water to the garden through a garden hose, is that the same as a dual system? No. A garden hose diverting raw laundry water to the garden is Tier 1 greywater diversion (simple, no approval, limited to same-day direct subsurface application). A formal dual system with an approved greywater treatment unit is a much more sophisticated installation that achieves higher treatment quality and allows broader reuse options.

Interested in a dual greywater/septic system for your rural Mid North Coast property? Contact us for a consultation, we can advise on whether your site is suitable and what the approval process involves.