The Black Caps had already won the opening T20I in Auckland on Friday by five wickets, before an emphatic nine-wicket victory at Hamilton’s Seddon Park on Sunday.
New Zealand restricted Pakistan to a total of 6-163, despite Mohammed Hafeez’s 99no, with Southee taking 4-21 from four overs, decimating the top order to 3-33 inside the opening six overs.
Hafeez’s rearguard offered something for the Black Caps to chase but Seifert made light work of the target, blasting 84no from 63 balls at the top of the order alongside Kane Williamson who made 57no from 42 deliveries. New Zealand won in the final over but with four balls to spare.
The knock was Seifert’s best-ever at international level, having played 29 T20Is for New Zealand, with five half-centuries to his name.
“It gives you that confidence that you can perform at this level consistently,” 26-year-old wicketkeeper-batsman Seifert said.
“I had a good start against India. Now it’s awesome I’ve kept on backing that up to know that I can perform at this level, rather than a couple of scores and you don’t do anything for a couple of years.”
The two sides will meet again in the final T20I on Tuesday in Napier, before turning to red ball cricket for a two-match Test series.
The First Test is in Mount Maunganui starting on Boxing Day, with the tourists yet to confirm their final Test squad from their large touring party.
Captain Babar Azam has missed the T20Is after a thumb injury and is in some doubt to be OK to play in the First Test.