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Kahui will head to Japan to play for the Toshiba club for two seasons at the end of this year's Super 15 season.
Hansen said today that All Black management were left in the dark over the Kahui's plans.
"We were very disappointed about that because he didn't really speak to us, or his agent didn't," Hansen told LiveSport radio.
"I always find that a bit frustrating."
The two-year Toshiba deal will enable Kahui to return to New Zealand to play in the Super 15 in 2015 and possibly push for a World Cup place.
Hansen said he spoke to Kahui following his decision and said he's taking a "big risk" if he wishes to wear the All Black jersey again.
"He's talked about coming back like everybody else but they take a big risk when they leave because there are other people here that have talent.
"He's obviously weighed that up and he believes he can come back so hopefully he does and hopefully he can come back and play at the level that gets him selected."
Kahui has played 17 tests for the All Blacks since making his debut in 2008 and has scored 10 tries.
The 27-year-old missed the international season last year after undergoing his fourth shoulder reconstruction.