Do not repot mid winter. There’s a reason why you repot in spring (and sometimes fall). You need two things for a successful repotting: metabolic activity and mild weather.
Spring is when the tree gets out of dormancy and the growth (auxin/cytokynin) cycle starts. It usually starts with root growth, as fine roots will inevitably become frost damaged and need healing, then the auxin moves to the branches where it drives foliar growth.
This presents an opportunity for repotting, as when your tree is actively growing roots, it will easily form a callus where you cut the roots and then grow new roots.
Fall is another possible period for repotting, because in the fall, with the loss of foliage, the tree focuses on vascular growth and that includes growing the root system. Weather conditions are also milder than in summer or winter, so the tree could still survive the operation.
However, repotting in the fall is riskier, because there’s only a brief window where that metabolic activity and vascular growth occurs, and your tree could be left with an underdeveloped root system and go into winter dormancy underprepared.
In winter, there are just so many things that could go wrong. The tree is already dormant, which means that it will most likely not form a callus, and in turn will not grow new roots. To add to that problem, its roots will have open wounds which can take in water and rot.
Now NZ winters are milder, but I’m sure it’s still not a smart choice to repot a fully dormant tree, for the reason I just mentioned. It’s not just about the cold and frost damage. Dormancy and the lack of metabolic activity is the real hindrance here.