There was more water this time around than there was in 1974, but the flood levels were higher in 1974 because there was no dam then.
Wivenhoe dam[1] did work as flood mitigation, but there were concerns during the flood as the water level was spiking quickly towards maximum storage[2], so some water was being let out (apparently about half of what was going in).
If the dam did get to full capacity its automatic system would have fully opened the flood gates, potentially making the flood situation quickly worsen even more.
See the interactive graph at the bottom of the page, you can choose to see only Wivenhoe's storage level history. Its level is almost back to 100% which is the normal water storage level, not the full flood mitigation level.
Wivenhoe dam[1] did work as flood mitigation, but there were concerns during the flood as the water level was spiking quickly towards maximum storage[2], so some water was being let out (apparently about half of what was going in).
If the dam did get to full capacity its automatic system would have fully opened the flood gates, potentially making the flood situation quickly worsen even more.
[1] http://www.seqwater.com.au/public/catch-store-treat/dams/wiv...
[2] http://www.seqwater.com.au/public/dam-levels
See the interactive graph at the bottom of the page, you can choose to see only Wivenhoe's storage level history. Its level is almost back to 100% which is the normal water storage level, not the full flood mitigation level.