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Flood-Prone Land
Floodplain Development Manual 2005 (NSW); various state guidelines; Council DCP/LEP flood chapter
When it triggers
Site mapped as flood-prone (1% AEP or PMF) under Council Flood Risk Management Plan or state flood mapping.
When it doesn't apply
Site not on flood mapping; or above 1:500 year flood (mostly outside FPL).
Report required
Flood Impact Assessment / Flood Compatibility Report
Produced by
Civil Engineer / Hydrologist (Engineers Australia)
Indicative cost
A$2,500–15,000
Key output fields
- 1% AEP flood level (RL AHD)
- Probable Maximum Flood (PMF) level
- Flood Planning Level (FPL) — typically 1% AEP + 500mm freeboard
- Hazard category (H1-H6, NSW classification)
- Floodway / flood storage / flood fringe classification
- Velocity-depth product (m²/s)
- Evacuation route + time
- Building floor level requirement
- Structural soundness above DFE
Design implications
- ▸FFL above FPL (usually 1% AEP + 500mm freeboard)
- ▸Habitable rooms above PMF (or compensatory storage)
- ▸No solid fences or obstructions in floodway
- ▸Pile/post or fillable foundations (not solid slab on floodway)
- ▸Electrical equipment above FPL
- ▸Garage access designed to drain back
- ▸Materials chosen for wet-dry cycling
Per-state notes
NSW: FDM 2005 + Considering Flooding 2022; flood prone land definition under FDM
VIC: Land Subject to Inundation Overlay (LSIO) + Floodway Overlay (FO) under Planning Scheme
QLD: Flood Hazard Overlay; State Planning Policy interim/permanent overlay
Notes
Flood reports often define the building envelope before architecture starts. Filling a flood-prone site usually requires compensatory storage. Floodway sites are highly restricted.