SYDNEY—A 174-foot-high dam is the only barrier saving Brisbane from an even bigger flooding disaster than the once-in-a-century deluge currently sweeping through the state of Queensland.

About 50 miles upstream, the Wivenhoe Dam, built in the wake of 1974 floods that damaged 6,700 homes in the city and killed 14, has rarely been tested over the past decade. Years of drought had left its reservoir drained. When the dam's spillway was opened in October 2010 to discharge excess water, crowds turned up to witness the first such release in 10 years. But the barrier is now operating close to capacity, raising fears that any increase in the floods flowing through local catchments could cause a catastrophic failure.

Officials from Southeast Queensland Water Grid who operate the dam didn't respond to questions because they were in the process of evacuating their office in Brisbane.

A 2007 report by Queensland's water commission on the dam and the smaller upstream Somerset Dam concluded that "a cascade failure of Somerset and Wivenhoe Dams is possible", though only in the event of a one in 100,000 year flood. The key risk to the dam is an overtopping, when floodwaters rise so high in the reservoir that they flow over the brim of the earth embankment and start washing away the structure of the dam itself.

To be sure, engineers familiar with the barrier argue that it's still well within safe limits. The 2007 report says the dam can discharge 950,000 cubic feet of water per second—greater than the volume of the U.S. Capitol building's dome in Washington, through a series of five 39-foot-wide floodgates.