The operator of Queensland's Wivenhoe Dam maintains it would have been "drastic" to pre-release water ahead of the 2011 floods based on predictions of extreme rainfall.
Seqwater has begun its defence in a class action in the NSW Supreme Court brought by 6000 flood victims whose properties were inundated by the huge amount of water released in January that year.
The plaintiffs claim the dam was operated negligently and engineers should have factored in rainfall predictions and released more water ahead of the peak to increase the available flood-storage capacity.
Seqwater's barrister, Brian O'Donnell QC, said that method was "extreme".
"Pre-releasing water from dams before the rain falls is a rather drastic thing to do," he told the court in Sydney on Friday.
Justice Robert Beech-Jones replied: "What if you were told there was a cyclone coming?"
Mr O'Donnell said if there was a large-scale rainfall event predicted to go beyond flood-storage capacity, pre-releasing water would be "a great idea".
"The bureau's advice has been you can't rely on the forecasts for making operational decisions," he said.
"That's the problem. You've got to look at a system that will work across a range of flood situations."
Mr O'Donnell cited evidence of the January 2011 weather system being isolated and unpredictable.
He also hit back at Maurice Blackburn's contention that engineers effectively "bet against" the Bureau of Meteorology forecast and said the alternative was to bet on the forecast coming true.
"Our case will be that the consistent advice from the Bureau of Meteorology and the flood engineers' own long experience in forecasts is that that is not a reliable thing to do," he said.
The court also heard operators had a responsibility to protect all of southeast Queensland from flooding - not just a select group of downstream properties
Mr O'Donnell said protecting the structural safety of the dams can require large releases to be made and insisted Wivenhoe was operated in a way that mitigated the severity of the flood.
The hearing continues.