Tasman maintained their hopes of a home semi-final by downing division leaders Counties Manukau 28-19 in a national provincial rugby clash in Pukekohe.
The Makos scored three tries to one in the all-Championship fixture to move to within a point of second-placed Otago with one round to go.
The top three teams are all guaranteed of making the play-offs, with the last spot to be fought over by Northland, Manawatu and Southland.
All Tasman's tries came from their wingers, Waratah Peter Betham getting a brace and Robbie Malneek marking in his 100th appearance for the region by grabbing the other.
The Makos had a strong scrum and dominated the breakdown exchanges, while Counties Manukau paid for a lack of accuracy at vital times.
The Steelers still have four-point margin at the top of the table, but they have now lost three in a row, although the two other defeats were against Premiership opposition.
They had the better of the opening exchanges on Tuesday night and a Tim Nanai-Williams penalty put them on the board early.
Tasman's response came against the run of play.
From a turnover, Malneek kicked ahead from deep in his own half, regathered and ran away to dot down.
The Makos then produced a well-worked backline move from a short line-out to put Betham over via Marshall kick-ahead.
Two further Nanai-Williams penalties meant the Steelers stayed in touch at 15-9 at half-time.
After the sides traded further penalties, the decisive score came midway through the second spell, with Marshall producing the break that set up Betham for his second touchdown.
The Steelers finally got over the line when lock Jimmy Tupou went over for a converted try, but couldn't get any closer over the final 15 minutes.
NZN