TP58 Wastewater FAQ — Coromandel Peninsula

TP58 is the New Zealand technical publication "Design of Onsite Effluent Disposal Systems". It is the primary standard for the design of onsite wastewater systems (septic systems) in New Zealand, covering site assessment, soil testing, and system design for properties not connected to reticulated sewer. Most district councils in New Zealand, including TCDC, require TP58-compliant design for onsite wastewater systems as part of building consent.

An onsite wastewater system (also called a septic system or onsite effluent disposal system) treats and disposes of wastewater generated by a property that is not connected to a public sewer. Most Coromandel Peninsula properties are in this category. A typical system includes a septic tank (or more advanced treatment unit) and a disposal field where treated effluent is soaked into the ground or otherwise dispersed.

A TP58 assessment is a site assessment and engineering report prepared by a suitably qualified practitioner (usually an engineer) in accordance with the TP58 standard. It includes site characterisation, soil permeability testing, and the design of a compliant onsite wastewater system. TCDC requires a TP58 report for building consent applications where onsite wastewater disposal is required.

TP58 goes beyond simply specifying a septic tank. It requires a systematic site assessment to understand the soil's capacity to safely receive treated effluent, followed by the design of an appropriately sized and sited disposal system. A "septic tank design" that doesn't address soil testing and disposal field sizing won't meet TCDC's TP58 requirements.

Yes — Thames-Coromandel District Council requires TP58-compliant wastewater design for all new buildings and additions requiring building consent where the property is not connected to the reticulated public sewer. Most Coromandel Peninsula properties outside the main town centres are not connected to public sewer and therefore require TP58 assessment.

Thames, Whangamata, and Tairua have reticulated wastewater systems for most of their urban areas, though even within these towns there are sections not connected. Hahei, Hot Water Beach, Waihi Beach, and Coromandel Town largely rely on onsite wastewater systems. In all cases, the specific connection status of your property should be confirmed with TCDC before assuming reticulated disposal is available.

Yes. The Coromandel Peninsula has significant environmental sensitivity — the Firth of Thames, Coromandel Harbour, and the many coastal estuaries are important ecological systems that are vulnerable to nutrient and pathogen inputs from poorly designed wastewater systems. TCDC and WRC apply close scrutiny to wastewater design near the coast, near streams, and in areas of groundwater sensitivity. Getting the TP58 assessment right is particularly important on the Coromandel.

A failing septic system on the Coromandel is a regulatory and environmental issue. If you observe signs of system failure (odours, surfacing effluent, wet patches, plumbing backup), you should address it promptly. TCDC may require formal remediation under the Building Act or Resource Management Act. A TP58 assessment by our engineers is the starting point for understanding the system's status and designing an appropriate solution.

TP58 permeability testing on the Coromandel typically involves hydraulic conductivity (permeability) testing in the proposed disposal area — usually using falling head permeameter tests or, for more complex sites, constant head or slug tests. The tests measure how quickly water moves through the soil at the depth where effluent will be disposed. This result, combined with the soil description and the system loading rate, determines the required disposal area.

Soil permeability on the Coromandel varies considerably by location: (1) Beach and coastal sands (Whangamata, Hot Water Beach, Waihi Beach) are fast-draining with high permeability; (2) Alluvial valley soils (Thames, Tairua valley) have moderate permeability; (3) Weathered volcanic hillside soils have variable permeability depending on clay content and whether they are naturally or artificially drained. Fast-draining sands are generally good for soakage disposal but may require additional treatment for pathogen removal. Slow-draining clay soils may require mound or elevated disposal systems.

TP58 requires a minimum separation between the bottom of the disposal system and the seasonally high groundwater table. On beachfront and low-lying Coromandel sections, groundwater tables can be very shallow — sometimes within 0.5m of the surface. This severely limits the depth at which conventional soakage disposal can be placed, and may require elevated disposal systems (mound systems) to achieve the required separation. Our site investigation always assesses groundwater depth as a key TP58 design parameter.

It depends on the site-specific conditions. Beach properties with deep, free-draining sands and a deep groundwater table may be able to use conventional soakage disposal. However, shallow groundwater (common near the foreshore), limited lot area, required setbacks from the coastal environment, and TCDC's effluent quality requirements in sensitive receiving environments may all constrain or preclude conventional soakage. We assess each site individually to find the right system.

Common system types on the Coromandel include: (1) Conventional septic tank + soakage field (most common where soils and lot size allow); (2) Secondary treatment systems (Biocycle, Puraflo, Orenco etc.) where higher treatment levels are required for the receiving environment; (3) Mound or elevated disposal systems where groundwater is shallow; (4) Pressure-dosed soakage where conventional gravity systems are not adequate. The appropriate system type for your site depends on the soil, the lot size, groundwater depth, and TCDC's receiving environment requirements.

Whether a secondary treatment system is required depends on your specific site and the receiving environment. TCDC and WRC may require higher treatment levels for properties near the coast, near sensitive waterways, or in areas where groundwater is at risk. Secondary treatment is more common on Coromandel beachfront and coastal properties than on inland lifestyle blocks. The TP58 assessment determines whether secondary treatment is required.

A mound system (or elevated soakage field) is a disposal system constructed above the natural ground level — essentially a raised bed of engineered fill that creates sufficient depth between the disposal pipes and the groundwater table. Mound systems are used on the Coromandel where groundwater is too shallow for conventional below-ground soakage. They are more expensive and visually prominent than conventional systems, but are often the only TP58-compliant option for low-lying sites.

Yes — the TP58 requirements apply equally to holiday homes and permanent residences. Any building consent for a new holiday home or significant addition on a property without reticulated sewer will require a TP58 assessment. The seasonal occupancy of many Coromandel holiday properties actually requires careful consideration in the design — peak loading in summer must be accounted for even if the house is empty in winter.

Building consent for significant renovations on the Coromandel often triggers reassessment of the wastewater system. Whether an upgrade is required depends on the age and condition of the system, the nature of the renovation, and whether additional bedrooms (and therefore additional wastewater loading) are proposed. TCDC processing staff will advise whether your specific renovation triggers a wastewater assessment requirement.

Composting toilets can reduce the blackwater (toilet waste) component of wastewater, but greywater (from sinks, showers, washing) still requires disposal. TCDC has specific requirements for composting toilet installations and greywater disposal that must be met. A partial TP58 assessment addressing the greywater component may still be required. We can advise on whether a composting toilet system is appropriate and viable for your specific Coromandel site.

Hot Water Beach presents specific challenges: small section sizes, high groundwater tables associated with the coastal aquifer and geothermal system, proximity to the foreshore, and TCDC's coastal environment rules. Wastewater design at Hot Water Beach requires site-specific testing and careful engineering. We have experience designing TP58-compliant systems for this type of challenging Coromandel site.

A TP58 assessment on the Coromandel Peninsula involves: (1) Site visit by our engineer to assess the site, inspect the existing system (if any), and conduct permeability testing; (2) Soil profile description (test pits or hand augers to characterise the soil to disposal depth); (3) Hydraulic conductivity testing (permeameter tests) in the proposed disposal area; (4) Groundwater depth assessment; (5) Design of the system (sizing, layout, setbacks); (6) Written TP58 report for TCDC building consent submission.

TP58 assessments on the Coromandel typically range from $1,200–$2,500 + GST for standard residential sites. Complex sites, those requiring extensive investigation, secondary treatment design, or mound system design cost more. Contact us with your site details for a specific quote.

We typically complete the site investigation within 5–7 days of engagement for Coromandel sites, with the written report delivered within 5–7 working days of the site visit. Urgent assessments can often be accommodated — contact us to discuss your timing.

Yes — this is our standard approach where both are needed. The soil investigation for TP58 and the foundation assessment for the geotechnical report share significant overlap. Combining them in a single site visit reduces your total cost and means the engineering is coordinated from a single team.