Blossom end rot is one of the most recognisable tomato problems: a dark, sunken, leathery patch at the base of the fruit, opposite the stem, that starts small and spreads as the fruit ripens. It's also one of the most commonly mistreated, because the standard home-gardener response, adding lime or a calcium supplement to the soil, usually doesn't fix it. That's not because the diagnosis is wrong, calcium really is involved, it's because the underlying cause almost never is.
Most garden soils, including most New Zealand soils, already contain more than enough calcium for a healthy tomato crop. Blossom end rot isn't usually a supply problem, it's a transport problem: calcium moves through a plant dissolved in water, carried along with it to wherever the plant is drawing moisture fastest. When watering is inconsistent, a dry stretch followed by a heavy soak, or hot, windy weather that outpaces a plant's roots, calcium gets pulled toward the leaves instead of the developing fruit, and the fruit itself ends up short despite the soil around it having plenty.
That's why blossom end rot often shows up in exactly the conditions you'd expect: container-grown tomatoes, which dry out faster and more unevenly than tomatoes in the ground, and gardens going through a run of hot days with irregular watering. It also tends to hit the first fruit of the season hardest, since a young plant's root system isn't yet established enough to keep pace with a sudden demand for water.
That's why blossom end rot often shows up in exactly the conditions you'd expect: container-grown tomatoes, which dry out faster and more unevenly than tomatoes in the ground, and gardens going through a run of hot days with irregular watering.
The actual fix is watering consistency, not calcium. Water deeply and on a regular schedule rather than in occasional large soaks, and mulch around the base of the plant to even out soil moisture between waterings. A foliar calcium spray can help as a short-term rescue on affected trusses, but it treats a symptom, not the cause, and won't prevent the next fruit from being affected if watering stays irregular.
Once a fruit shows blossom end rot, there's no fixing that particular tomato. The plants around it will keep showing the same pattern until watering evens out, which is exactly the kind of thing that's obvious in hindsight and easy to miss in the moment: premium check-ins on this site track a plant against its own recent watering and weather pattern, so a dry stretch gets flagged as a real risk before the next truss of fruit shows the damage.