Maintaining Your OSD System: A Post-Handover Guide

The system you forget about until it floods

An on-site detention system is unusual among building assets: it does its job perhaps a handful of times a year, invisibly, underground or disguised as a lawn. That makes it spectacularly easy to neglect — until a blocked outlet turns a designed detention tank into a permanent pond, or an overflowing pit sends stormwater through a garage in the first big storm of summer. OSD obligations don’t end when the occupation certificate is issued. Here’s what owners, strata schemes and builders each need to know about life after handover.

Maintenance isn’t optional — it’s on the title

Almost every OSD system in NSW is protected by a positive covenant registered on the title under the Conveyancing Act 1919 (created via an 88B instrument on subdivided land, or by separate dealing otherwise). The covenant legally requires the owner — every owner, forever — to keep the system in working order, and gives council the power to enforce. In practice that means council can require rectification, and in serious cases carry out works and recover costs. The covenant follows the land: a buyer who’s never heard of OSD inherits the obligation at settlement, which is exactly why the covenant exists.

What actually needs doing

The good news: OSD maintenance is simple. The whole system has one critical vulnerability — the small outlet that controls flow — and most maintenance exists to protect it.

Routine (a few times a year, and after big storms)

Clear the trash screen. Leaves, litter and silt collect on the screen protecting the orifice. A blocked screen stops the tank draining; a missing screen lets debris block the orifice itself. This is the single most important task on the list.
Check the orifice plate. It should be present, fixed and unobstructed. It should never be removed or enlarged “to help the water get away” — that defeats the entire system and breaches the covenant.
Look into pits. Grates clear, no sediment build-up, no ponding that suggests a downstream blockage.
Walk the surface storage. For basin systems: mow it, keep the outlet clear, and don’t let landscaping, sheds or soil regrading eat the storage volume.

Periodic (every year or two)

Remove accumulated silt from tank floors and pit sumps.
Check structural items — lids, steps, covers — remain sound and secure. Below-ground tanks are confined spaces: internal entry is a job for appropriately trained contractors, not a Saturday job.
After any landscaping, paving or renovation work, confirm nothing was connected into, built over, or regraded away from the OSD system.

For strata schemes: the OSD is common property

On duplex, townhouse and apartment developments, the OSD system almost always sits on common property — which makes the owners corporation responsible. Practical advice for strata committees:

Put OSD on the scheduled maintenance program alongside fire services and lifts — typically an inspection and clean once or twice a year through the strata manager’s contractor.
Keep the WAE survey, design drawings and covenant terms with the scheme’s records so contractors know what they’re maintaining and where it is.
Budget for occasional silt removal in the capital works fund. It’s cheap compared with one flooded basement.

For builders: handover is where neglect starts (or doesn’t)

Builders shape whether a system survives its first decade. Three habits worth adopting:

Hand over the documents. Give the owner or strata manager the approved stormwater plans, the works-as-executed survey, the engineer’s certification and a plain-English note on what the system is and what to clean. An owner who doesn’t know the lawn is a detention basin will eventually landscape it out of existence.
Leave the system clean. Construction is the dirtiest period of an OSD system’s life — wash-down silt, render, sawdust and packaging end up in pits. Jet and clean the system before handover, not just before the certifier’s visit.
Show the client the screen. Five minutes at handover pointing out the discharge control pit and trash screen prevents years of ignorance. Some councils require an OSD marker plate or signage at the pit — make sure it’s installed.

The cost of neglect, in one paragraph

A neglected OSD system fails in two directions at once. Blocked, it stops protecting your own property — water backs up, surcharges where it shouldn’t, and finds the garage. And non-functional, it stops protecting the catchment, putting the owner in breach of a covenant that council can enforce. Ten minutes with a gloved hand on the trash screen a few times a year prevents both.

Contrive Consultants designs council-ready OSD and stormwater systems across Sydney and NSW, and can review, certify or troubleshoot existing systems. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us today.