"For turtles, their shell is actually part of their body, and shell damage had exposed some of the bone which leaves a high risk of infection.
"It's still in a critical condition, which means it has a 50-50 chance [of survival]."
The turtle, which weighs 25kg, was now resting up in a temperature controlled tank.
"What they are doing to measure its temperature is measuring the temperature of the pool they are keeping it in, which is being kept at a constant temperature.
"They would usually be found in temperatures that are 25C to 27C, so as long as the pool is being kept at that temperature the turtle will be in a much more comfortable environment than when it was first found."
X-rays taken as part of yesterday's health assessment showed the turtles lungs were clear.
"The lungs are clear, but pneumonia is really hard to detect in the early stages so they have administered some antibiotics to ward it off essentially.
"Reptiles do take a long time to recover from anything that is going on, so it will take a little while before we know what is going to happen."
Ms Callander said it was rare to see a turtle in such cold waters at this time of year.
"Even in the past New Zealand hasn't seen many of these turtles on its shores, they're usually much further north."
She said they were unsure as to how the turtle ended up in New Zealand waters.
"We think that it might have been caught in a strong current that it wasn't able to swim out of. That may mean it may have had an underlying health issue already, or it might just mean that it is a juvenile turtle that just wasn't strong enough to get out of the current."
Vets at Wellington Zoo believed the turtle was female, and would name her Ridley, as she was an olive ridley sea turtle, Ms Callander said.
Ms Callander said vets would continue to try and stabilise the turtle's condition, before working with other conservation agencies to organise rehabilitation and release.
"We don't have the facilities to get it rehabilitated -- we can get it medically fit and we can look after it at that point, but once it's going well it will need to be in a warm water pool so it can swim around and we just don't have that at the zoo."