The university said they had mistakenly cut down several trees without approval from the National Capital Authority, which required an application before tree felling was allowed.

"The University wrongly believed the NCA in early January had given approval for the tree removals," an ANU spokesman said.

"Once ANU became aware of the mistake, the University informed the NCA of the error. ANU has halted work on that element of the building project until works are approved. The University is continuing to work with the NCA over the approvals."

"The trees were white poplars and English elm, and they will be replaced by new oak trees and Red Box eucalypts."

The NCA deferred any questions to the ANU who said the approval documents for the works were still with the NCA but under discussion.

The works to remove the trees were under a separate approval lodged with the NCA and it was not made clear how the mistake happened.

In public consultation documents for the current works, one submission noted the university had seemingly pre-determined the outcome of the trees in their promotional material.

The NCA responded documents for the works concerning the trees would be made publicly available.

The $220 million Union Court redevelopment has been bogged down with problems. Last year, the student union took the ANU to the ACT Supreme Court over concerns about where they would be relocated considering their existing offices were marked for demolition.

The ANU Union and the university reached a private agreement but would not provide much detail on what it entailed.

The university also confidentially settled with Gods Cafe owner Jaye Min, after he lodged a lawsuit against the ANU, arguing his lease had been illegally terminated.