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The part-time left arm spinner also picked up his maiden one-day wickets (two for 55) as Otago's chase was extinguished for 275.
It was the home side's fifth consecutive loss in the tournament and has left its playoff prospects in a precarious position.
The Volts will need a bonus-point win in their last round-robin game against Auckland at Eden Park Outer Oval on Sunday and must hope Wellington beats Canterbury but misses out on a bonus point.
Northern, meanwhile, secured its playoff berth with the 66-run win.
Volts coach Rob Walter felt his side leaked too many runs in the final 15 overs and thought a score of 270 was about par.
''We were on track [to restrict Northern] to do that but just did not bowl well enough at the death,'' Walter said.
''Daryl Mitchell showed us what was required. He took a lot of pace off and used a lot of variation.
''And to be fair Kelly batted really well.''
Mitchell was useful with two for 42 but the day belonged to Kelly.
He was particularly brutal on Michael Rae, clouting the Volts paceman for three sixes as he raced to his hundred off 86 balls.
The spinners all went for at least one six with Kelly swatting seven in total.
He finally ran out of luck on 106 - his second one-day ton - when he mistimed a shot to long-off. But by then Otago was looking at an enormous chase.
Northern's tally was the highest one-day total by a domestic side at the venue. The Black Caps hold the ground record with 360 for five against Sri Lanka in 2014-15.
The Australian-born Kelly got good support from Dean Brownlie, who scored 56.
Former test opener Daniel Flynn came in at No8 and thrashed 37 not out from 13 balls, while BJ Watling created a nice platform with 40 at the top of the order.
Anaru Kitchen was the pick of the Otago bowlers with the left-arm spinner taking one for 45 from nine overs.
Otago's chase suffered an early setback when opener Hamish Rutherford was out hooking for four.
It is a shot the left-hander did not play much earlier in his career and his dismissal continued a lean sequence of 21, five, nought, 25, six and four in limited-overs fixtures this year.
Shawn Hicks (39) and Neil Broom added 54 for the second wicket, while Kitchen put in a 30-minute shift at the crease before he dragged a delivery back on to his stumps on 27.
They were two big partnerships to break but the key moment came when Broom also chopped on soon after for 39.
Brett Hampton made the triple strike and it left Otago's middle order with a mountainous task.
Derek de Boorder and Michael Rippon kept swinging. But their partnership of 83 runs from 84 balls, as good as it was, did not keep pace with the required run rate, which had ballooned to nearly 14 runs by the time Rippon was trapped lbw for 37.
Mark Craig (22 off 13) helped plunder 25 off the 45th over but his side needed 70 off the last four overs and fell short.
Otago skipper Rob Nicol sat out the match with an infection in his Achilles. It is unclear whether he will be available on Sunday. Jimmy Neesham captained the side.
In the other games, Canterbury beat Auckland by 38 runs in Christchurch to move into fourth place.
Competition leader Central Districts downed Wellington by three-wickets in a tense match at the Basin Reserve.