Over the past two seasons Cullen has established herself as the Ferns first choice wing defence - a role Harrison has filled on occasion.
Southby said she is wary about rushing Harrison straight back into action.
"I'd like to think we won't need to rush people back in and players get out on court because they have earned the spot out there. Anna certainly does bring something different in that wing defence role and it's nice to have her back in the group for that," said Southby.
"The physios and strength and conditioning staff worked really hard to make sure the programmes we had [Harrison and Tutaia] progressing on were as close as possible to what we do in camp. I don't know if we'll be waiting with baited breath, but we're obviously really keen to see how they come through that first training tomorrow. There's a plan in place for them to do that so we can use them through the series, but at this stage we are still guided by the medical team."
Harrison, who has not played for the Ferns since the 2014 Commonwealth Games, is desperate to hit the court following her false start to the international season.
The 33 year-old defender was ruled out of the Quad Series on the eve of the opening clash against England and has watched on as rookie defender Jane Watson excelled in her absence. Harrison said the pressure to quickly regain her best form was already on before Cullen's withdrawal.
"I put that pressure on myself before knowing about Kayla's injury. I want to come in here and hit the ground running. I'm here because I want to be here and want to be performing at my best, I don't just want to cruise and come along for the ride."
Harrison rates herself at being about 90 per cent off her top form. She said physically she is up for the challenge of taking on the world champions, but she is still seeking to regain confidence and momentum.