Otago's McCullum cracked 116 not out off just 56 balls, including 12 fours and six sixes, as New Zealand registered their highest Twenty20 innings total at AMI Stadium.
He was just one run short of the West Indies' Chris Gayle's record score of 117 against South Africa in Johannesburg two seasons ago, and McCullum equalled Gayle's record 50-ball century.
The innings, highlights by two lap shots over wicketkeeper Brad Haddin for six off speedster Shaun Tait, topped McCullum's own New Zealand record of 69 against India last season.
He and Gareth Hopkins (36) whacked 68 off five overs for the fifth wicket, while McCullum helped take 71 off the final five overs.
New Zealand's previous highest total was 198 for five against Ireland at Nottingham last year.
Tait ended with two for 40 off four overs, while fellow paceman Dirk Nannes had one for 51 off four.
Australia -- who won the series opener by six wickets -- rested two of their best players, Mitchell Johnson and Shane Watson, and replaced them with Ryan Harris and Travis Birt.
