Key, who four years ago scored a hundred in each innings of Kent's nine-wicket win over New Zealand, ended the first day of this three-day game with an unbeaten 178 against his name in a total of 324 for one.
Runs came from the other end too, with James Tredwell 123 not out.
New Zealand, whose captain and star spinner Daniel Vettori is one of five members of their squad currently taking part in an Indian domestic Twenty20 tournament, begin a three-Test series against England at Lord's on May 15.
The 28-year-old Key, who played the last of his 15 Tests three years ago, completed his century in 170 balls with 16 fours.
Tredwell, whose score was only his second first-class hundred, reached three figures off 198 balls with 17 boundaries.
New Zealand teenage quick Tim Southee, who took five wickets on his Test debut against England at Napier in March, returned figures Monday of one for 49 off 15 overs although it needed a fine catch by James Marshall at backward point to help him dismiss Joe Denly for 12.