The former Silver Fern is contracted with the Lightning until the end of the 2019 season, and with the Lightning in a playoffs push, her season won't conclude until August 5 (if the Lightning miss the playoffs) or the end of August (if they make the final).
Adding to the complication is the Netball NZ claim they have not been in contact with any prospective candidates.
"We've had a head coach in the role up until today and it would have been incredibly unprofessional and inappropriate to make any other approaches," said Netball NZ CEO Jennie Wyllie.
With a short turnaround to sign a coach before their next squad is named, Wyllie admitted a caretaker coach could be an option.
"Time is of the essence - we need to be realistic, and flexible. That might mean putting in a caretaker coach. We need to work through that process in the next few days," said Wyllie.
Southby's resignation leaves a clean slate with the Ferns' coaching staff. Her assistant coach for the Commonwealth Games, Yvette McCausland-Durie, is now off-contract and is unlikely to put her hand up for the role due to being under contract with the Central Pulse for next season.
"I would assume you couldn't have that role if you were in a head coach role at a franchise anyway, so I had never looked at that. I signed up last year for two years [with the Pulse] and I feel that's a really important step for me in my development. I've got lots still to learn," she told Radio Sport.
McCausland-Durie says she would be more likely to push for a role as coach of the newly-formed New Zealand A side, or try to stay on as assistant.
"What I've had with the assistant coach role has been really significant, and if there's something around that role - it might work."