The new Cotter Dam already contains about six times the volume of water that the old dam could hold after recent rainfall increased the ACT's overall storage levels by more than 4 per cent.
Despite now containing more than 24-billion litres, the new Cotter Dam still has some way to go before it reaches its 78-billion-litre capacity, which will increase the overall ACT water storage capacity by 35 per cent.
ACTEW Water managing director Mark Sullivan said in a statement: ''We're delighted that our dams are now storing more water than ever before, ensuring we can deliver water security for future generations.''
Cotter Dam is now almost a third full, four days before the celebration to mark the completion of the project.
Titled Cotterfest, the public is invited to visit the site, with Q&A sessions on the dam as well as its official opening by chief minister Katy Gallagher.
As parking is extremely limited at the site, ACTEW Water is asking people to park at Stromlo Forest Park and catch a free shuttle service to the dam site, which will be open from 9am to 4pm.