The projects are part of a $9 million public art spending spree by the City of Sydney.

The centrepiece, an arch to "frame" the Town Hall and stand over George Street, will be composed of white stainless steel and is intended to evoke the image of a cloud.

When privately unveiled to councillors earlier, one was heard to compare the design to a silhouette of Casper the Friendly Ghost.

A brief provided to the media states it will stand between 50 and 75 metres high. Lord Mayor Clover Moore described the design as "awe-inspiring".

The piece is designed by 37-year-old award-winning Japanese artist Junya Ishigami, who describes his style as "architecture as air" and takes inspiration from weather patterns.

"The cloud is that new symbol, comprehensible to all," the council said. "The cloud evokes comfort, openness and freedom.

"Through freedom it is connected to cloud computing which in turn links to the quality of a connected city."

Ishigami, a critic-in-residence at Harvard University, was announced as the winner of another architecture competition last week in Copenhagen.

His winning design was a floating cloud shelter for the harbour.

"I thought, 'wow'," Cr Moore said of her initial reaction to the arch's unveiling. "People know the Opera House, they know the Harbour Bridge and in future they'll know the cloud.

"It will be up there with the other great structures in the world."

A second "whimsical" installation will feature an oversize milk crate, which will stand in Belmore Park near Central Station. Designed by Egyptian-born Hany Armanious, the dark blue crate will stand 13.7 metres high.

The third design unveiled was 60 handmade bronze bird sculptures for the underpass of Kent Street, designed by British artist Tracy Emin.

"Taken together, the three will activate the centre of the city from north to south and east to west," Cr Moore said.

The artworks were chosen by a panel of public art experts appointed by the city. Despite being closely guarded by the city, many details of the installations were published ahead of time by Fairfax Media. Cr Moore said keeping the selection process secret was necessary but that the designs would be placed on the city's website for comment. "Today we begin that public conversation," she said.

Tim Williams, the head of the advocacy group the Committee for Sydney welcomed the installations as a commercial boon for the city centre: "We're going to get hundreds of thousands of visitors," he said. "Ten years form now we'll have people marrying by the [milk] crate, they'll be making love in the arch.

"We'll look back and think why did we make a fuss about this thing?"

The sculptures are expected to be completed within three to seven years and will be timed to coincide with the completion of the light rail project on George Street.

Name the sculpture

SMH is challenging readers to come up with the best nickname of Sydney's new artwork. Make a suggestion in the comment stream below - and we'll put the best entries to a vote later on Tuesday. Or join in on Twitter at #namethesculpture