Cotterfest's carnival atmosphere with face-painting, balloons and ice-cream was popular with the younger visitors and for many amateur photographers it was a chance to document an awesome man-made construction.

Lance Scaife-Elliott, 42, brought his two sons Jack, 9, and Forrest, 8, into the bowels of the dam structure to see water monitoring instruments, outlet valves, drainage pipes and two sets of 295-step stairs leading to the top.

''They have been on school excursions to see the outside so it's good to be able to show them the inside,'' Mr Scaife-Elliott said.

The extension increased water storage capacity from four gigalitres to 78 gigalitres, although its cost has, at times, caused concern.

Since the project began, its budget has climbed from $363 million to $409 million.

ACTEW managing director Mark Sullivan was on hand with Chief Minister Katy Gallagher to officially open the enlarged dam and celebrate the launch of a 2.5-kilometre walking trail from the valley floor in Cotter Avenue to the top of the new dam wall.

''Canberra is a growing city and the investment in this major piece of infrastructure will ensure that the community has a secure and ongoing water source for years to come,'' Ms Gallagher said.

''I am also pleased to see the environmental benefits this project will provide with measures to protect endangered fish species such as the Macquarie perch.''

Mr Sullivan said that he was not sure when the underbelly of the dam would be open for public viewing again. ''For us, it has been a day to celebrate that we are handing back the dam to the people, handing back the Cotter, and our engagement to this dam has come to a pretty good conclusion,'' Mr Sullivan said.

''I don't think we'll be doing it [open days] that often. Logistically, it's big and they require us to convince a lot of our staff to voluntary work for the day.

''I think we'll open the dam regularly - maybe yearly we'll do something.''