Even in polite company, it was a dam fine day for a historic walk

Many other sightseers were turned away when the open day proved too popular.

Canberra's water supplier, ACTEW Water, gave residents the opportunity to peer over both sides of the Cotter Dam's giant, 80-metre-high walls, watched on by a team of safety officials.

All of the tours were booked out before lunchtime.

The utility's managing director, Mark Sullivan, acted as a guide for the day, taking busloads of visitors to the top of the dam.

''It is genuinely a once-in-a-lifetime experience. Clearly, people are very interested and the weather has been kind,'' he said.

''Unfortunately, not every one will get to see this, because, without safety personnel, we just can't let people onto the crest of the dam.''

The new dam, which is mostly finished, holds almost 20 times as much water as the one it replaced, and has expanded Canberra's water supplies by more than a third.

Poor weather delayed the construction several times, and the opposition has regularly criticised the project's costs, which have almost tripled to $410 million.

However, Mr Sullivan said it was one of few urban water supply dams built in Australia in recent decades and was the fastest-built ''by a long, long way''.

''The dam will be judged well in history. When you spend money, and when you spend more money than you hoped to spend, you are judged critically and that's fair enough,'' he said. ''But the next drought will come, and we'll say, 'Thank heavens we did this'.''

A range of people took up the invitation on Sunday, from families with babies to ''dam junkies''.

Mick Tunbridge, of Holt, and Heather Gibson, of Dunlop, have attended every open day since construction began in 2009, watching the project unfold.

Mr Tunbridge recalled that, as a child, he often played in the catchment.

''Isn't it magnificent? I've been looking forward to today … I would never have missed the chance to be up here.''

Carol Vickers, of Garran, and her daughter, Phoebe, 13, arrived to see what Ms Vickers called ''part of our history''.

''I guess this is a historic moment, in a way, and I've been wanting to look at it up close,'' she said.

Canberrans might not have another chance to walk atop the dam wall, but Mr Sullivan said ACTEW Water would convert the truck haulage roads into walking tracks and create an observation and picnic area near the top. A discovery trail and viewing platform have already been built near the base.

''We can't not utilise what will be a magnificent reservoir,'' Mr Sullivan said.

''We'll make sure people can look at it.''