Mostert and a three-wicket haul by Stephan Grobler helped North Otago to a comfortable win over rival South Canterbury in a Hawke Cup warm-up match on Sunday.
South Canterbury won the toss and asked North Otago to bat in the 60-over match at Centennial Park, a decision that looked to be a winner after the home side was reduced to 70 for five in the 32nd over.
North Otago initially looked solid after an opening partnership of 32 between Jeremiah Shields and Lachie Kingan, before the latter was the first to fall, for 19.
The wicket of debutant Cormack Hassett for 10 with the score on 54 and three more in quick succession, a period during which North Otago lost four wickets for just 16 runs, put the visitors on top.
It left the home side searching for a steadying influence.
That came in the form of the ever-reliable Mostert, who stuck a composed 52 from 80 balls which featured six fours and a six in what would later prove to be a vital knock.
However, he had little support from the lower order with the exception of Brady Kingan, who chipped in with a grafting 13 off 40 balls.
When Scott Kitto was the last man to fall for six, North Otago had 158 on the board – a total it would have been content with, considering its earlier collapse.
Chasing the modest total, South Canterbury got off to the worst possible start when Mostert removed Sam Carlaw for a four-ball duck.
Much as North Otago had done earlier, the visitors found themselves in something of a hole at 73 for five, thanks to some disciplined bowling and sharp catching.
The game was all but over when South Canterbury lost three wickets with the score on 115, a blow it was not able to recover from. It was bowled out for 137 in the 46th over.
Grobler was the best of the North Otago bowlers with three for 17, including a double-wicket maiden, from six overs.
He was well backed up by Scott Kitto who claimed two for 12 and Jeremy Smith with two for 28.