It is poised to unveil a major public artwork for George Street in the coming weeks, designed to be the centrepiece of the city's oldest street when it is finally turned into a pedestrian boulevard, shorn of cars and home to a new tram route.

Some councillors are comparing the expected public reaction to its unveiling in the same breath as the reaction to the Eiffel Tower.

Exactly what plans lord mayor Clover Moore and her colleagues at the council have been looking at has been a closely held secret. But intriguing details are now leaking out.

At a council meeting on Monday, Liberal councillor Edward Mandla, who opposes the secrecy surrounding the project, alluded to a structure reaching from ''Woolworths to the QVB'', and which might stand for 200 or 300 years.

The Herald learnt on Tuesday from a source familiar with the plans that they involve a spiralling steel archway stretching diagonally across the street with a peak that could rise higher than the Town Hall, which stands 60 metres tall.

After hundreds of artists expressed interest in designing a centrepiece last year, a panel of art experts has unanimously endorsed a design, which a council committee voted to recommend.

But what that design is is intended to remain secret until it is formally adopted next week.

Cr Mandla threatened to go public about the proposal on Monday and so inspired debate about how democratic decisions about public art can or should be reached.

''I was told the public rarely likes public art but they grow fond of it over time,'' Cr Mandla said of a discussion with a council official.

Councillor John Mant, who is aligned with Cr Moore, said it made no sense to open the public art proposal to attacks from ''shock jocks''.

''There will be a public reaction as there was when the Eiffel Tower went up,'' he said. ''We should be strong enough and proud enough of our process to say, 'This is what we’re going to do.' ''

But Cr Mandla argued the finalists' designs should have been open to public comment because of the scale of the installation.

His threat to go public also drew a threat.

''[I] would be obliged to report any matters of suspected corruption to ICAC (Independent Commission Against Corruption),'' the city's chief executive, Monica Barone, said. ''If confidential matters were disclosed to the media that would likely come within the definition of corruption.''

A spokeswoman for the city said proposals needed to be kept secret to protect the commercial position of people who bid for its projects.

She said the council had budgeted about $6.5 million for three George Street artworks and expected them to be completed along with the light rail project later this decade.

When approved, the proposal is expected to go through the normal development approval process, which includes being placed on public display.