A police bomb squad is in place to inspect the device.
The package, addressed to a member of staff, was handed over in a carrier bag at the opera’s reception desk.
The man responsible is known to staff at the opera house and is reported to have acted in a threatening manner before.
Opera spokesman Marcus Gorme confirmed that Monday’s events could be described as part of a relationship drama.
“One could say that,” he said.
The suspicious object has been removed from the building and placed on the river bank.
While the opera house has not been evacuated, staff are being advised to stay away from the windows.
Staff sensed that something was not quite right after reading a message written on the package. Police have not revealed the the content of the message.