Pakistan chasing 265 to win shortened third New Zealand ODI
Captain Michael Bracewell also surpassed 50 as the home side set a competitive total in their bid to sweep the three-match series.
The contest was reduced to 42 overs each after a wet outfield at Bay Oval delayed the start of play by nearly two hours.
Opener Mariu, playing just his second international match, scored a composed 58 off 61 balls, featuring six fours and two sixes.
A number of New Zealand middle-order batsmen made starts but didn't press on for big scores until Bracewell produced a flashy 59 at the death.