Wivenhoe Dam will release 116,000 megalitres into the Brisbane River, which flows through the state capital, from Saturday.
Water Minister Glenn Butcher used his ministerial powers to authorise the release amid a higher forecast risk of tropical lows, floods and cyclones in coming months.
Premier Annastacia Palaszczuk says lowering the dam level from 90 per cent to 80 per cent capacity will ensure it can be more effectively used for flood mitigation.
"We've all seen what has been happening with devastating flooding in NSW and Victoria. Tasmania is about to experience a significant flood event," Ms Palaszczuk told parliament on Thursday.
"We must be ready for the worst.
"From Saturday, SEQWater will begin the release of water from Wivenhoe Dam in an early move to prepare for the worst-case scenario based on the (Bureau of Meteorology) advice."
The decision follows a report into the February-March floods in which 13 people died and more than 9000 homes and businesses were damaged, finding Wivenhoe had been "well-managed".
Brisbane Lord Mayor Adrian Schrinner has been calling for water to be pre-emptively released in recent months, but the government dismissed his pleas as being too soon.
Mr Butcher said on Thursday he had authorised the release in response to the BoM's long range flood forecast.
He's the second water minister to authorise a dry-season release of drinking water from Wivenhoe in state history.
"This isn't a decision that I've taken lightly," he said.
"We saw in February that these extreme weather events can be unpredictable and keeping Queensland safe is at the top of my priority list."
Ms Palaszczuk also announced that 13,200 litres of free water, about two week's worth, would be given to households across southeast Queensland connected to the SEQWater grid.
The report into the preparations for and response to the 2022 floods by the Inspector-General of Emergency Management was released on Wednesday.
It found the overloaded State Disaster Coordination Centre failed to issue timely alerts in Brisbane and some properties were inundated without warning.
Some local government staff also didn't have enough training or knowledge to effectively use the state-run alert system.
Inspector-general Alistair Dawson called for a new process for alert requests requiring "urgent approval and distribution without delay" to be implemented by November.
He also recommended a review of the process, including requesting, composing, authorising and issuing messages by the same date.
A probe of the Queensland Emergency Alert Manual should also be conducted and completed by November next year, the report said.
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