Their 31-16 win means they finish second on the log and will host the Tasman Makos, who have never beaten them at Eden Park, next Friday night.
Canterbury, who host Taranaki on Saturday, are top qualifiers and have home advantage through the finals.
Auckland, if they can address their scrum concerns and tighten up the breakdown area, where the Steelers gave it all, will be favourites to do the business in their first home semifinal since 2010.
The key man was right wing Worcester-bound Bryce Heem. He scored a hat-trick, including two solo tries, and ran with vigour and elusiveness all night.
An increasingly desperate Auckland lifted their game in the second stanza, gradually hauling themselves back onto the contest and then the lead as Counties Manukau incurred the wrath of referee Paul Williams, conceding a clutch of penalties.
Replacement halfback Jono Hickey assumed the kicking duties off his brother, captain Simon Hickey, and duly slotted three from four off the tee.
Heem's second try emanated from second five George Moala who, in partnership with Charlie Piutau, created space for Heem.
Earlier in the second half, Steelers No 8 Maama Vaipulu scored his second try from a solid lineout drive, and the home side looked like they would not let the initiative go. But they could not contain the irrepressible Heem.
The Steelers played some fine football in the first half. They looked like men unshackled, as they have been for the past four games. Working off a dominant scrum, where Pauliasi Manu continues to do damage to opposition tightheads, in this case Ofa Tu'ungafasi, and with a tigerish loose trio getting the better of Auckland at the breakdown and with ball in hand, they led 8-5 at oranges but it could have been more with the amount of possession they had.
They also set up their mauls well, and Vaipulu scored their first try off a lineout drive, the umpteenth try from such a mode this season.
Openside flanker Sam Henwood, one of six current or former Auckland University club men in the game, two from the Steelers, twisted and turned but lost the ball over the line.
Auckland's first score was a solo try from Heem, his second individual effort in as many weeks, as he sought to erase memories of his Ranfurly Shield horror. First five Simon Hickey missed a penalty goal sitter when he lost his footing in the act of kicking the ball.
Auckland lost loosehead prop Sam Prattley, stretchered off after what looked an innocuous incident out of a maul. He was sent to hospital.
Counties Manukau 16 (M. Vaipulu 2 tries; P. Francis 2 pen) Auckland 31 (B. Heem 3 tries; S. Hickey con, pen, J. Hickey con, 3 pen) Halftime: 8-5
- by Campbell Burnes
