Gardiner has recently left the Bears after two and a half seasons in charge, and he has also been an assistant coach of the New South Wales under-20s side.
"Obviously this highlights the calibre of the coach Garth is with other organisations also wanting his services," said Kidwell. "So as disappointed as we are not to have him on board, we're really happy for him, and we are really happy to have Ben to join our team."
Kidwell and Gardiner worked together in 2010 with the South Sydney Under-20s team.
"Ben and I have a long relationship, he's a good man and will bring a real focus on our defence" , said Kidwell. "He's done an excellent job everywhere he's been and I'm looking forward to his contribution to the Kiwis.'
"Thanks to all the forward planning we've done in regards to this World Cup campaign, Ben as already made a seamless transition into camp."
Gardiner, who has also had an assistant coach role with the NSW Under-20s side is thrilled with his promotion.
"It's a privilege to be in the Kiwis and to contribute to improving the players and the team's direction," said Gardiner. "I look forward to learning more about the New Zealand culture and adding my personality and determination to compete.'
Gardiner arrived in camp on Wednesday.