He will join Northampton coach Wayne Smith in the supporting role to New Zealand coach Graham Henry.
Smith is not expected in New Zealand until May as Northampton are still chasing the play-offs in the English Premiership.
Hansen said: "I am delighted to confirm my acceptance of the position as assistant coach to Graham Henry with the All Blacks.
"The appointment will begin on my return to New Zealand after the Six Nations and I am looking forward to working with Graham again and my old mate Wayne Smith."
Hansen insists he remains totally focussed on his role with Wales and the upcoming game against Ireland and will make no further comment at this time.
Welsh Rugby Union chief executive David Moffett added: "Steve Hansen has told me that he remains completely focussed and committed to Wales' Six Nations programme.
"The New Zealand post is not something that will be exercising his mind until he returns home.
"This appointment is not totally unexpected and I am now considering sending an invoice to the NZRFU chief executive Chris Moller on behalf of the WRU for playing this important role in the development of two of their coaches.
"I do hope that everyone will respect his wishes in not raising any questions about the All Blacks job until after this week's important match."