During the first spell, Manawatu were able to play the percentages as they worked the corners via Black's accurate boot and backed their forwards to make their tackles.
Milner-Skudder's individual brilliance led to the game's first try, while Black ticked the scoreboard over with penalties as he passed the 100-point mark for the season.
With a 13-3 advantage at the break, Manawatu were the first team to dot down in the second spell when Gibbins, who has signed an upgraded two-year contract with the Hurricanes, produced a 40m solo effort to score.
Southland have mixed their form all season and made the Championship playoffs despite a record of four wins, five losses and a draw.
Lima Sopoaga has been their lynchpin in 2014 but Black had the better of the duel between the No 10s as Sopoaga wasn't able to take control of the game like his opposite did.
The Stags looked out of puff at 23-6 down but No 8 Elliot Dixon found his way to the tryline as the visitors fought their way back in to a game that involved plenty of niggle. Referee Ben O'Keeffe was busy as he tried to keep both sides calm throughout the contest.
Manawatu enjoyed a mountain of territory in the final quarter but they were unable to find a knockout blow as the Stags defended stoutly.
A late Southland try gave Manawatu the jitters but the home side were able to hold on, which would have pleased coach Jason O'Halloran.
Manawatu will host Hawke's Bay on Friday night in a repeat of the 2011 Championship final. The winner will replace the relegated Wellington in the Premiership next season.
Manawatu 23 (Nehe Milner-Skudder, Callum Gibbins tries; Otere Black 3 pen, 2 con) Southland 18 (Eliot Dixon, Keanu Kahukura tries; Lima Sopoaga 2 pen, con) halftime: 13-3