Limits
What this cannot tell you
Most sites put this in small type at the bottom. It belongs at the top, because the gap between what Watermark observed and what it means for your house is where somebody could get hurt.
This is not a warning service and never will be. Everything here is in the past tense. If water is rising now: call 000 in an emergency, call the SES on 132 500 for flood and storm help, and get current warnings from the Bureau of Meteorology.
A clean record is good news. It is not a guarantee.
This is the most important sentence on the site. If Watermark says water has never been observed on your ground, what that means precisely is: no clear satellite pass in forty years caught water there.
Landsat passes over any given place about every eight to sixteen days. A flood that arrived on a Tuesday and drained by Thursday was seen by nothing. Floods also arrive with weather, and cloud hides the ground — so the passes during an event are exactly the ones most likely to be unusable. Watermark counts those as "obscured" rather than "dry", and shows you how many clear looks it actually had, but it cannot fill the gaps between them.
Read a clean record as "nothing was caught here in forty years of fortnightly looks", not as "this has never flooded". The first is what the data says. The second is a much bigger claim that no satellite can support.
A 30-metre pixel is not your house
Each observation covers a square about 30 metres on a side — roughly the footprint of a large house and its yard. Within one pixel there is no distinction between a flooded driveway and water over a floor. Flooding stops at fence lines and kerbs that a satellite cannot see.
This is why the site also looks at the ground around you and will say things like "no water on this exact spot, but water has been observed 100 metres away" — which is usually more useful than either half on its own.
A river gauge is not your street
Where Watermark pairs an observation with a gauge reading, that reading describes the river at the gauge, which may be kilometres from you and separated by terrain this site does not model.
Gauge heights are also measured against local datums. A reading of 10 metres at one station has no relationship to 10 metres at another — they are not heights above sea level and they are not comparable between stations. That is why a level is only ever shown against the station it came from, and never converted or compared across stations.
The watch level is an observation, not a threshold
Where the record supports it, Watermark reports the range between the highest gauge reading on a day this ground was seen dry and the lowest reading on a day it was seen wet. Somewhere in that range, water first reached that ground.
It is built from a handful of observations, not from a hydraulic model. Rivers change: channels shift, levees are built, works are done upstream, and gauges are moved or re-levelled. A range derived from 1990s observations may not describe the river today. Treat it as a number worth watching on the Bureau's own gauge page, not as a line below which you are safe.
Heights are approximate
Ground heights come from a national elevation model with an error of a few metres in many places. Watermark will tell you that a road out dips five metres below your ground — a difference large enough to be real. It will not tell you that your floor sits 200 mm above your neighbour's, because nothing here can know that.
What water does is not ours to say
Where this site reports a height difference, that difference is a measurement. What water actually does with it depends on how a flood behaves across a catchment — whether the water surface is level across the area, where it enters from, what is upstream. That is hydraulic modelling. Watermark does not do it and will not imply it.
Absence of data is not absence of flooding
Sometimes a source cannot be reached, a gauge has no reading for a date, or a place is outside a dataset's coverage. The site is built to say so explicitly rather than to render a blank or a zero. "We could not ask" and "we asked and there was nothing there" are different facts, and if you see one of them stated, it is because it is true — not because something failed quietly.
Not advice
- Not a flood certificate. If you need a formal flood search for a purchase, planning application or insurance claim, your council or state authority issues the document that counts. This is not it.
- Not legal, financial, insurance or property advice. Nothing here is a recommendation to buy, not buy, insure, or price anything.
- Not a substitute for a flood study. Council and state flood studies model events this record cannot observe, including events larger than anything that has happened in the satellite era.
- Not official. Watermark is independent. Where its figures differ from your council's, the council's are the ones with legal standing.
No warranty
This site is provided free and as-is, with no warranty of accuracy, completeness or fitness for any purpose. It joins two independent public datasets by date and location, and both have edges. Decisions about property, safety and money should not rest on it alone. To the extent the law allows, no liability is accepted for any loss arising from its use — and separately from that, if a number here is wrong I would genuinely like to know: siroj.8b@gmail.com.