Mr Alexander said the tenant had commissioned an engineer, and he was in the process of having his own report done to assess the safety of the building.
He understood the building was fine internally, after a major rebuild about two years ago.
"Most of the issues are external. I believe it is to do with unreinforced brickwork, and the brickwork on the outside of Brunner House is contained in concrete.
"From what I understand, contained brickwork is vastly safer than uncontained and in Christchurch during the big earthquake buildings that did not have contained brickwork, the issues were severe."
Having endured the Christchurch quakes, Mr Alexander said he understood why the decision had been taken to move out.
"It is unnerving and the West Coast is being proactive getting us to do the work before something may happen."
In the long run, it had to be better for everyone, "except from an economic point of view, it will be harder on landlords".
ACC began reviewing the earthquake safety of all its offices after the February 2011 earthquake. That involved checking council registers for listings of buildings that were earthquake-prone, as well as obtaining seismic ratings of buildings from landlords, or by commissioning their own engineer's reports.
ACC today apologised for the disruption, and stressed that the move was made to ensure the safety of staff and clients.