Kahui has suffered a series of shoulder problems and the 27-year-old centre and wing said last week's decision to sign a lucrative two-year contract with Toshiba Brave Lupus in the less intense Japanese Top League was made with his injury record in mind.
Currently with the Waikato Chiefs but out of action after further shoulder surgery, Kahui will move to Tokyo at the end of the Super Rugby season and Hansen was unhappy the move had been arranged without his knowledge.
"We were very disappointed with that because he didn't really speak to us, or his agent didn't," Hansen told New Zealand's LiveSport breakfast show on Tuesday.
"I always find that a bit frustrating.
"Normally you hear from them, then you've got an opportunity to discuss it, and put a reasonable offer and if you want them to stay you've got an opportunity to fight to keep them.
"In this case we didn't so that was a bit disappointing. But we've discussed that and moved on."
Kahui's move means he will not be eligible to play for New Zealand but the player said he would return after completing his contract in Japan and try to force his way back into the All Blacks squad ahead of the 2015 World Cup in England.
The deal is another feather in the cap for the domestic game in Japan, which has managed to attract some of the sport's biggest names to ply their trade in a league dominated by backs.
World Cup-winner Sonny Bill Williams collected a reported $1 million for his stint with the Panasonic Wild Knights at the end of last year, while former Wallabies flanker George Smith led the Suntory Sungoliath to the title last month.
Kahui's All Blacks team mate Carter could also make the move north, according to reports in New Zealand media.
Carter, the world's best flyhalf and twice named IRB player of the year, was set to sign a short term deal with a Japanese Club in excess of $1 million following the conclusion of New Zealand's November internationals, various media outlets said.