An Italian consumer group said Saturday it suspected artist Christo's popular floating walkway on a lake had involved an unreasonable waste of public money.

The organisation Codacons said it would be filing a complaint to the Lombardy region's spending watchdog on Monday.

"The Floating Piers" has been a smash hit with the public with visitors flocking to experience the three-kilometre-long (1.9-mile) walkway of 200,000 floating cubes on Lake Iseo since it opened on June 18.

But according to Codacons, the huge cost of cleaning up after tens of thousands of visitors and ensuring their safety made it questionable whether the art installation should have been authorised.

"We want to know how much taxpayers' money has been spent on a project which, until now, seems to have generated enormous publicity for the artist without bringing direct benefits to local entities and citizens," the organisation said in a statement.

Organisers of the installation this week announced they would have to close the walkway at night because it is being worn out faster than expected.

The numbers of people who have come to walk on the free installation have so far exceeded organisers' expectations, who estimated around 500,000 would visit over the 16 days and nights it was due to be open. Some 270,000 turned up in the first five days.

Bulgarian artist Christo, 81, is famed for works on a massive scale, including wrapping the Berlin Reichstag in fabric in 1995.