With the wet season only just underway, it might make some a little nervous to hear south-east Queensland's biggest reservoir – Wivenhoe Dam – is just about full. 

But the people in charge of the dam say water levels are exactly where they need to be.

Located north-west of Brisbane, Wivenhoe Dam is one of 26 dams operated by Seqwater – and is by far the biggest of the lot.

When deemed 'full', the dam can hold just under 1.2 million megalitres.

In comparison, the next largest – Somerset – is around a third of the size.

Why Wivenhoe?

Wivenhoe Dam provides drinking water and flood protection for much of the state's south-east.(Supplied: Seqwater)

Completed in 1984, Wivenhoe Dam was built with the primary purpose of providing drinking water to the millions of people who live in south-east Queensland.

Its secondary function is to mitigate flooding by controlling the flow of both the Brisbane and Stanley Rivers which pass through some of the heaviest rainfall areas in the country.

Data on flooding in the city of Brisbane dating back to the 1890s shows in that regard, it has been very successful.

But the operators of the dam warn that even with the best management strategy in place, Wivenhoe will not hold back the next major flood.

How much water can it hold?

A search online for 'Wivenhoe dam level' will give you a percentage somewhere between 80-90 per cent.

However, to use official jargon, that means "operational supply level" and is a percentage of its total drinking water supply level, not its actual capacity. 

That means that during a flood event, Wivenhoe can hold far more than 1.2 million megalitres.

The flood operations centre uses complex software to monitor rainfall and river conditions.(ABC News: Will Murray)

Seqwater's principal dam safety and operations manager Kevin Bartlett said this is thanks to an additional compartment located above the drinking water.

"It's double the size," he explained.

"So, there's another two million megalitres of flood storage available."

In a flood, instead of that additional water rushing downstream as it would naturally, the dam allows Seqwater to release it in a more controlled manner through Wivenhoe's five gates.

When are the floodgates opened?

Which gates to open, when to do so, and by how much, is determined by engineers based at Seqwater's flood operations centre, which is based in Brisbane.

They rely on an array of hundreds of sensors dotted across the catchment which provide real-time data on rainfall, run-off, flow rates and dam levels.

Forecasts from the Bureau of Meteorology are also used to model how a flood event will play out.

Wivenhoe Dam's floodgates are used to control the flow of water during rain events.(ABC News: Will Murray)

Wivenhoe's floodgates are used to control the flow of water during rain events.

Hundreds of sensors arranged across the catchment provide data on rainfall, runoff, flowrates and dam levels.(ABC News: Will Murray)

Hundreds of sensors arranged across the catchment provide data on rainfall, run-off, flow rates and dam levels.

Weather forecasts from the Bureau of Meteorology are also taken into account when deciding which gates to open.(ABC News: Will Murray)

Weather forecasts from the Bureau of Meteorology are also taken into account when deciding which gates to open.

As the dam fills above operational full supply level, the engineers direct that the gates start to open in accordance with the Wivenhoe flood manual.

"[The manual] effectively sets all of the processes we follow – how we release, when we release," Mr Bartlett said.

"Once we get only a very small margin above that operational full supply level, we must actually open gates."

Floodgates are opened to control the dam's capacity during severe weather events.(Supplied: Seqwater)

The lessons of 2011

After the 2011 floods, the operators of Wivenhoe were accused of breaching their duty by failing to follow the flood manual.

In 2019, the Supreme Court of New South Wales found Seqwater and others had been negligent – essentially by not releasing water soon enough.

Seqwater later won an appeal against that decision, successfully arguing that the senior flood manager at the time followed a legitimate strategy.

Severe flooding in 2011 led to the reviewing of the dam's processes for dealing with floods.(Supplied: Angus Veitch)

However, it prompted several reviews of the flood manual and explains why today's engineers are so devoted to its stipulations.

The floods of 2011 also prompted political calls to drain Wivenhoe to below full supply level ahead of forecast heavy rain.

But the manual does not allow for that, and in fact requires that the dam be kept as close to full supply as possible.

Seqwater CEO Emma Thomas says the primary function of Wivenhoe Dam is to provide drinking water.(ABC News: Will Murray)

Seqwater chief executive Emma Thomas explained that is because forecasts can be unreliable and the next drought could be just around the corner.

"Our primary purpose is to provide drinking water to south-east Queensland to over 3.7 million people," she said.

"We have to provide that water to them."

The Wivenhoe myth

Even if operated in full accordance with the manual, operators say Wivenhoe will not be able to stop a major flood from affecting those downstream.

That's because only the upper Brisbane and Stanley Rivers are controlled by the Wivenhoe and Somerset dams.

Wivenhoe and Somerset dams control the upper Brisbane and Stanley rivers, but not Lockyer Creek, Bremer River or the lower Brisbane River.(ABC News: Will Murray)

Other waterways, including the volatile Lockyer Creek, Bremer River and lower Brisbane River, are all unregulated.

"We only can control half of the overall catchment to Brisbane,"

Mr Bartlett said.

"So, flooding can occur naturally."

During a particularly severe event, dam operators lose some of the limited control they have, because Wivenhoe is designed to spill over once it reaches a certain level to prevent a catastrophic structural failure.

Dam operators have been conducting maintenance ahead of summer.(ABC News: Will Murray)

However, when possible, controlled releases are timed so as not to make that naturally-occurring flooding worse.

"[During ex-tropical cyclone Alfred] we were able to wait for the peaks to come up in those downstream catchments and to pass through before we started to increase our releases from Wivenhoe," Mr Bartlett said.

Stay alert, be prepared

Dam releases regularly cause Brisbane River crossings to be impassable, and dangerously charge downstream waterways.

Ms Thomas said Seqwater notifies local governments and disaster management teams as early as possible regarding releases.

But she warned advanced notice was not always possible.

"We're all part of the community as well and would like as much notification as possible," Ms Thomas said.

"But I think we all understand that our weather is very changeable, and we all need to be able to react and respond."

Posted 2 hours agoMon 29 Dec 2025 at 7:34pm, updated 2 hours agoMon 29 Dec 2025 at 7:39pm