Drainage diagram in a NSW contract: how to read the sewer service diagram

Most NSW contracts for the sale of land include a hand-drawn or scanned plan of the property with lines and arrows running through it. That is the drainage diagram, usually a sewer service diagram from the water authority. It is easy to skip. It is also one of the few documents in the contract that tells you what is underground, which matters if you plan to extend, add a pool or build a granny flat.

This guide explains what the diagram is, how to read it, and what questions to ask. It is general information, not legal advice or plumbing advice.

Why it is in the contract

NSW Fair Trading lists "drainage diagrams" among the documents that must be attached to a contract for the sale of residential land under section 52A of the Conveyancing Act 1919 and Schedule 1 of the Conveyancing (Sale of Land) Regulation 2022. If a required document is not attached before the contract is signed, the purchaser may be able to rescind within 14 days of exchange.

In Sydney Water's area, the document is a sewer service diagram. Elsewhere, it comes from the local water and sewerage authority, which may be a council or another water utility.

Sewer service diagram versus service location print

Sydney Water issues several documents that buyers and conveyancers see. Two are often confused.

Sewer service diagram. Sydney Water describes this (also called a property sewerage diagram or sewer diagram) as showing the private wastewater pipes on a property. Buildings and pipes are shown, but not pools. Agents and conveyancers use it to check whether plumbing work on the property has been inspected and whether any private pipes cross the property. Sydney Water notes that NSW Fair Trading, and now Building Commission NSW, have inspected private plumbing work since 2012; before 2012, Sydney Water did.

Service location print. This shows Sydney Water's own pipes and structures on a property and the property's point of connection to the wastewater system. Sydney Water notes the point of connection may be on another property.

The contract usually includes the sewer service diagram. Your conveyancer may also obtain a service location print.

How to read the diagram

Diagrams vary in age and quality. Some are decades old and hand-drawn. Look for:

1. The outline of the buildings. Does it match what is there now? If the house has been extended and the diagram shows the original footprint, the extension's plumbing may not be recorded.
2. The private drainage lines. These run from fixtures (toilets, showers, kitchen) to the connection point. They are your responsibility as owner.
3. The authority's sewer main and the junction. Where does the property connect? Is the main within your lot, along a boundary, or on a neighbour's land?
4. Pipes crossing from or to neighbouring lots. A private line serving another property that runs through your land may need an easement. Check the title documents.
5. Dates and inspection notes. Dates help you match the diagram against building approvals.

Why it matters for building

A sewer main running through the backyard is common in older suburbs. It does not stop you buying, but it can limit what you build.

Sydney Water says it must approve all buildings and structures near or over its wastewater pipes that need approval from council or a private certifier. It issues a "building over sewer letter", which tells you whether it approved a building close to or over its pipes. Agents and conveyancers use it to check whether a building on a property was built with Sydney Water's approval.

Practical questions if the diagram shows a main near the house or an addition:

• Was the extension, garage or deck built over or next to the main approved? Ask whether a building over sewer letter is available.
• If you plan to build near the main, expect requirements for clearances and access, and potentially costs. Get advice before you commit.
• Is the main protected by a registered easement? In many older areas it is not, but the authority's rights can still apply.

See easement on title for how registered easements appear in title documents.

Warning signs

• The diagram does not match the house. Extensions, second bathrooms or a granny flat not shown.
• No diagram, or a note saying none is available. Ask the vendor's solicitor what searches were made.
• A pool sits over or near a line. Sydney Water notes pools are not shown on the diagram, so check separately.
• Private lines from neighbours. Ask whether there is an easement and who maintains the pipe.
• A septic system or "not connected". Unsewered properties have different maintenance and approval requirements.

Stormwater is a separate question

The sewer diagram is about wastewater. Stormwater, the water from roofs and hard surfaces, is usually the council's concern. Stormwater may drain to the street, to an easement through neighbouring properties, or to an on-site detention system. On-site detention tanks are often covered by a positive covenant on title that requires you to maintain them. Check the registered dealings attached to the contract and the planning certificate for flood-related controls.

How ReadyToSign helps

ReadyToSign locates the drainage diagram in your NSW contract, notes whether one is attached, and compares what the papers say about sewer, stormwater and any septic system. It also flags easements and positive covenants on title that relate to drainage. It cannot read pipe positions reliably from every hand-drawn diagram and will say so when a page is too faint. It is not plumbing or legal advice.

Get a report on your NSW contract

Related: What to look for in a contract of sale in NSW · Section 10.7 planning certificate

Frequently asked questions

Is a drainage diagram required in a NSW contract?

Yes. NSW Fair Trading lists drainage diagrams among the documents that must be attached to a contract for the sale of residential land. Missing documents may allow the purchaser to rescind within 14 days of exchange.

What does a sewer service diagram show?

According to Sydney Water, it shows the private wastewater pipes on a property, including buildings and pipes but not pools. It is used to check whether plumbing work has been inspected and whether private pipes cross the property.

Can I build over a sewer main in NSW?

It depends on the authority's requirements. Sydney Water says it must approve buildings and structures near or over its pipes that need council or certifier approval. Get advice before planning work near a main.

What is a building over sewer letter?

A Sydney Water letter that tells you whether it approved a building close to or over its wastewater pipes. Conveyancers use it to check whether an existing structure was approved.

Does the diagram show stormwater?

The sewer service diagram is about wastewater. Stormwater arrangements are usually shown elsewhere, such as in council records, easements or positive covenants on title.

Sources

• NSW Government (NSW Fair Trading), Sales contracts – requirements for property agents: https://www.nsw.gov.au/housing-and-construction/property-professionals/working-as-an-agent/sales-contracts
• Sydney Water, Certificates, documents and diagrams: https://www.sydneywater.com.au/accounts-billing/managing-your-account/buying-and-selling-a-property/certificates-documents-diagrams.html
• Conveyancing (Sale of Land) Regulation 2022 (NSW): https://legislation.nsw.gov.au/view/whole/html/inforce/current/sl-2022-0485

General information only, not legal advice. Last reviewed 18 October 2026.