However, New South Wales Swifts coach Julie Fitzgerald is believed to have registered interest in the role, and Invercargill's Natalie Avellino also remains a strong candidate.
Southby has not been returning calls, and Steel chief executive Julie Paterson said she was not able to discuss individual candidates when asked if Southby had been interviewed on Saturday.
However, most of the signs point to a two-horse race between the Dunedin-based Southby and former Australian international Avellino, with the franchise understood to be reluctant to appoint an overseas coach at the expense of a good local candidate.
The former shooter made more than 30 appearances for Australia and, in 2008, had success as a player-coach when she led Southland to its first national title in 49 years. The following season Southland came second with Avellino at the helm again.
But, when asked, the New Zealand Schools assistant coach acknowledged Southby had better coaching credentials.
"Well, she is the New Zealand Under-21 coach, so she is well ahead of me," she responded.
However, Avellino felt her interview went reasonably well.
"It's hard to know how you go in interviews. Some areas I'm obviously a little bit inexperienced but, overall, it went well.
"We'll just have to wait and see."
Fitzgerald, who has also been linked with the Central Pulse role, is a vastly experienced and successful coach.
Southby, who played for the Western Flyers before shifting to Dunedin to captain the Otago Rebels in 2002-03, started her coaching career as assistant to Dame Lois Muir in 2005. She took over as coach of the Rebels in 2006 and had two seasons before the team was disbanded.
Southby then served an apprenticeship as assistant coach of the New Zealand Under-21 team in 2008 and 2009 before taking over as head coach last year.
Paterson said the interview panel consisting of herself, Muir, New Zealand high performance manager Tracey Fear and Steel board member Doreen Evans interviewed six candidates in Invercargill on Saturday, but a final decision had not been made.
"We are just working our way through the process at the moment," Paterson said.
'I would like to think we would have something finalised by the end of this week or early next week."