He said: "This is a tragic event that could not have been reasonably foreseen and we are deeply saddened by it."
Mr Holleran told journalists that Mel Greig and Michael Christian's show would not be returning until further notice.
But he added: "I think that prank calls as a craft in radio had been going on for decades. They are done worldwide and no-one could reasonably have foreseen what happened."
The network later suspended all advertising on 2Day FM until Monday.