Rachin Ravindra (79) and Mark Chapman (62) smashed attacking half-centuries to power the hosts to a commanding 9-255 after a rain delay reduced the match to a 37-overs-a-side contest at Seddon Park.
Sri Lanka slumped to 4-22 inside six overs and were eventually bundled out for 142 in 30.2 overs as New Zealand took an unassailable 2-0 lead in the series.
"Can't complain, we opened up pretty well with the bat at the start," New Zealand skipper Mitchell Santner said.
"I think it was still very competitive and we knew we had to bowl well at the start."
Put in to bat, New Zealand lost Will Young for 16, but player-of-the-match Ravindra and Chapman combined in a second wicket stand of 112 to lay the foundations for a big total.
Daryl Mitchell made a run-a-ball 38 but New Zealand squeezed out only 63 runs from the final 10 overs as Theekshana (4-44) led Sri Lanka's fightback.
The off-spinner removed Santner and Nathan Smith with the last two deliveries of his seventh over.
He then returned to dismiss Matt Henry with the first ball of his eighth to become the seventh Sri Lankan bowler to take an ODI hat-trick.
Sri Lanka wobbled early in their chase after Jacob Duffy struck twice and touring captain Charith Asalanka was run out in the fifth over.
Kamindu Mendis struck a brisk 64 before falling to Will O' Rourke, who claimed 3-31 to lead the rout of the visitors.
"It's a small ground and it was a fair pitch. I think bad batting in the powerplay cost us the game," Asalanka said.
New Zealand will be eyeing a series sweep in the third ODI in Auckland on Saturday.
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