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.
The skipper struck six sixes in his 40-ball knock before being caught off the last ball of the innings bowled by Akif Javed.
Seamer Javed took 4-62 off his eight overs although Naseem Shah was arguably the best of the Pakistan bowlers, taking 2-54 and bowling with good pace and movement.
Pakistan will attempt to salvage some pride with the bat in the final match of an unsuccessful tour.
They lost the first ODI in Napier by 73 runs and the second in Hamilton by 84 runs, having been defeated 4-1 in the T20 series.