The pair were appointed co-coaches last year but the Steel reverted to a more traditional structure for this season. Southby assumed sole charge, and Avellino was her assistant.
But tension between the former equals had obviously grown. Netball South chief executive Sue Clarke described the relationship as ''no longer tenable''.
''Both Janine and Natalie agree the coaching relationship is no longer tenable and the decision has been made for Natalie to finish with the franchise,'' Clarke said in a statement.
''Ultimately, it is my responsibility to have the Southern Steel operating in the best environment possible as we compete in the 2013 ANZ Championship season. This is where our energy is focused and nothing can be allowed to detract from this.''
Avellino was terribly disappointed but guarded in her comments.
''To be honest, it is hard for me to comment,'' she told the Otago Daily Times.
''I am bound to some confidentiality, so all I can go with is what was in that press release.''
The press release was not particularly revealing. She was quoted as saying; ''I have been very proud to represent Southland''.
''I was looking forward to furthering my coaching and believe I had a lot to offer but felt restricted in the opportunities.''
Avellino is expecting her first child in June and that is her immediate priority.
''That will be my focus and we will wait and see where things go from there. I loved being involved with the Steel and coaching at that level.''
Southby was more forthcoming when contacted for comment.
''We have different philosophies and it wasn't conducive to a healthy team environment,'' she said.
''We operate in different ways and it was just getting to a stage where it wasn't working and it was pretty challenging for both of us.
''The both of us worked pretty hard at trying to make it work. We had good support systems in place but it got to the stage where it wasn't working.
''The team has been briefed and I guess there is some surprise from some of them.
''Going forward, there will be different support structures in place from a coaching perspective for me. There are a couple of options that I'm looking at, at this stage.''
Clarke confirmed the franchise was considering replacing Avellino and was working through options with Southby and Netball New Zealand.
Clarke was not prepared to discuss what financial agreement was reached with Avellino or what impact that would have on the Steel's finances.
Senior player Wendy Frew said the players had been taken by surprise when they were given the news yesterday morning.
''It is a shock for us,'' Frew said.
''From what I know, the players did not see it coming. I guess Janine and Nat have been really professional in terms of putting the team first... and did not want it to affect the team.
''It is all pretty new... so the group has not had a big discussion about it. For me, I didn't really see it coming so I haven't really had time to digest it.''
