Tasman appeared to blindside Counties, who didn't expect such a spirited showing from the amalgamated union made up of Marlborough and Nelson Bays.
Counties fullback Tim Nanai-Williams was forced in to the goalkicking duties after Steelers pivot Baden Kerr was taken from the field in the opening stages with a suspected concussion.
Nanai-Williams came in to the game having only made two from nine attempts off the tee this season but he was a perfect three from three in the opening 40 minutes as he kept the home side in the game.
The Steelers were given a ray of hope in the final quarter when lock Jimmy Tupou scored his first ITM Cup try and Nanai-Williams added the extras, but nine points was as close as they could get to a red-hot Makos outfit.
There was a momentum-stopping break in the 67th minute of the game when referee Garratt Williamson had to leave the paddock with an injury; he was replaced by James Imlach who was given the chance to control an ITM Cup game for the first time but he looked assured with the whistle.
Counties, who still sit atop the Championship table despite suffering their third loss of the campaign, round out the regular season on Saturday afternoon when they meet Northland in Whangarei.
Tasman 28 (Peter Betham 2, Robbie Malneek tries; Tom Marshall 2 con, 3 pen)
Counties-Manukau 19 (Jimmy Tupou try; Tim Nanai-Williams 4 pen, con) HT: 15-9