Police closed Knowles Place, just off London Circuit, to cars and pedestrians and restricted some access to the ACT Magistrates Court and Supreme Court about 1pm.

CFMEU ACT secretary Dean Hall said the crane's driver inspected the machine after he noticed a shudder. The site was later evaluated by Lang O'Rourke site management.

An experienced engineer was travelling from Sydney to inspect the crane in the evening.

The site and crane would remain closed and out of action until they were cleared by the engineer. Extra parts of the precinct were fenced off in the afternoon.

A WorkSafe ACT spokeswoman said in a statement a prohibition notice had been issued on the crane until the engineer inspected the machine and made further recommendations.

Construction workers had been told to leave the site.

"There is no immediate risk to safety of workers or the community however as a precaution some roads in the immediate area have been closed temporarily and traffic diversions are in place," the spokeswoman said.

Mr Hall said it was unacceptable that the ACT "continues to be the least safe state or territory in Australia".

"This is an extremely serious incident that again highlights the need for the highest possible safety standards to be enforced on ACT construction sites.

"The CFMEU will continue to fight for workers to return home to their loved ones and also to keep the public safe from the dangers of construction sites."

Building works on the $150 million upgrade to the ACT's ageing court infrastructure began in April last year. The revamped courts are expected to meet the territory's needs for the next 50 years.