McMillan, 37, played 55 Tests and 197 one-day internationals for New Zealand between 1997 and 2007, and since his retirement has worked in both commentary and coaching. He was the batting coach for Canterbury when they won the Plunket Shield last season and worked with New Zealand during the Wellington Test against West Indies in December when Carter was absent.
Lindsay Crocker, the head of cricket with NZC, said McMillan was the right man for the job. "We had a number of strong applicants from New Zealand and abroad, but overall we felt that Craig was the best fit for the role," Crocker said.
A full-time batting coach will be named after the end of the West Indies tour. New Zealand fly to the Caribbean in late May for three Tests and two Twenty20 internationals.