Crane hanging 'precariously' from 51st floor of Barangaroo tower

An exclusion zone is still on force at the bottom of Tower One at the northern end of the building project, but there were no reports of injuries in the incident which occurred at about 2pm.

The 3.9-tonne spider boom crane involved in the incident was bolted onto the top of Tower One, and was lowering a so-called boom lift at the time.

Police have closed Hickson Road and Sussex Street to traffic and pedestrians between Erskine Street and Towns Place.

A NSW Police spokeswoman said early on Wednesday morning that the operation to remove the crane was ongoing, and was expected to take until later in the morning at least.

"We're asking people to avoid the area," she said.

The closure of the roads due to the crane breakdown caused traffic delays on Wednesday. The 311, 324 and 325 buses were diverted away from Hickson Road, while the 288, 290 and 292 services started their trips from King Street Wharf instead of Sussex Street.

The Traffic Management Centre said it did not know when Hickson Road would be able to be opened.

A spokesman for the Construction, Forestry, Mining and Energy Union, which represents workers at the site, said the crane was "hanging precariously" from the top of Tower One.

"They were lowering pieces of machinery and it has sort of collapsed under that load," he said on Tuesday. "As to the cause, it is hard to tell at the moment. It is up 51 floors."

Tower One has a small number of tenants who recently moved in.

Lend Lease, the developer of Barangaroo, said in a statement on Wednesday that it was continuing to work with a team of crane experts on a process to safely lower the load and remove the crane from the roof of Tower One.

"The precautionary exclusion zone remains in place this morning with traffic management along the closed section of Hickson Road. We will advise next steps in due course," it said.

The latest incident follows the dramatic collapse of a crane on a Meriton building in north Sydney in September, which left three workers dangling from the high rise.

CFMEU state secretary Brian Parker said he understood a mechanical failure had led to the incident at the top of the Barangaroo tower.

"This is the second one we have had in a short space of time [in Sydney]," he said. "Work is going to be stopped [on Tower One] until such a time that we are satisfied that no one is going to be put at risk."

Mr Parker said he was concerned that too many incidents were occurring during the erection and dismantling of cranes, and the union would "be in touch with SafeWork and we will be saying that we will have a blitz".

Construction of Tower One, which is now one of Sydney's tallest buildings, is close to completion.

However, it will eventually be overtaken by Crown's casino and hotel complex, which will be built just north of it at Barangaroo.

With Megan Levy