Retallick had already been ruled out of the first test of the three-match series at Eden Park on Saturday with what was initially thought to be a pectoral muscle injury.
Hansen, however, said Retallick had actually suffered a fracture during the Waikato Chiefs' Super Rugby loss to the Canterbury Crusaders last week.
"He hasn't got a pec tear, which is great news, but he has got a small fracture - I'm not even going to attempt to tell you what bone it is," Hansen told reporters in Auckland after he had named his side for the first test.
"It's right in underneath his pec. He'll probably out for six weeks."
Retallick is the fourth senior All Blacks player to be ruled out of the entire series with injury, with captain Kieran Read (back), hooker Dane Coles (knee) and inside centre Sonny Bill Williams (knee) all missing from the squad.
Hansen said it was possible Retallick would be fit for the Super Rugby playoffs, which begin on July 21. The southern hemisphere's Rugby Championship starts on Aug. 18 with the All Blacks playing Australia in Sydney.
(Reporting by Greg Stutchbury; Editing by Peter Rutherford)
