A South Korean city is asking people to submit ideas for improving a 34.4bn won (£18m) observation tower that was so badly received it has never been allowed to open.

Changwon, an industrial port city on the south coast, built the 40-metre Big Tree in Daesang Park as a new symbol for itself.

When it was unveiled last year, residents complained that what had been built looked nothing like the renderings they had been shown.

The design had called for a 20-metre artificial tree, costing 2bn won, to sit on top of the structure, but it was dropped from the plans, leaving the bare steel frame that stands there now. The city has said the change was made for public safety, over the risk of the artificial leaves falling, and that the money saved went on lighting instead.

Locals have called it the “balding tree” and the “shame tree”. During a two-week trial opening last August, 85 per cent of 1,868 people surveyed rated its appearance negatively, and some called for it to be pulled down. The city postponed the official opening, which had been set for 1 October, indefinitely.

The tower was built under a scheme in which a private developer is allowed to build flats on part of a public park in exchange for laying out the rest of it and handing it to the city. In this case the developer built 1,779 flats on about an eighth of the park's 96 hectares.

The row has since moved beyond how the tower looks.

Changwon audited the project between February and May, examining whether the design change had been properly reviewed, and concluded that officials and the developer's supervisors had not carried out those checks properly.

Five council officials were disciplined and the city asked police in June to investigate two people on the developer's side, over the possible inflation of construction costs.

The city has declined to publish the audit findings, saying they contain material that could affect the police inquiry and any future litigation. The speaker of the city council, Son Tae-hwa, told a press conference in June that the council had pressed for the audit for months and had still not been given the results, saying the city's reason was that reporting to the council would make the findings public.

The contest, which runs until 2 September and is open to anyone in South Korea, is seeking practical proposals on the structure's design, safety, convenience and possible uses. Winning entries are expected to be announced in September and may feed into a redesign.

A city official said it had not yet been decided how much would be spent on any improvements, and that even on the fastest timetable the Big Tree is unlikely to open before next year.