"Based on information from the Fire Service and police we're confident the substance won't turn out to have been hazardous - but we have to treat the situation as though it were," he said.
"Staff who were directly affected dealt with the situation very well but it was traumatic for them.
"Specialist cleaners have decontaminated the mail area and immediate vicinity and we'll keep air conditioning off in that part of the building until we know more about the substance. If it's found to be hazardous we will get air conditioning specialists to clean the system.