A citrus tree dropping leaves, sometimes a scatter, sometimes what feels like most of the canopy over a couple of weeks, is alarming to watch but rarely a single, simple problem. A handful of causes account for most cases in a New Zealand garden, and they're distinguishable enough, once you know what to check, to narrow down fairly quickly.
Watering, in either direction, is the most common cause by a wide margin. Citrus genuinely dislike wet feet: soil that stays waterlogged, whether from overwatering or simply poor drainage, suffocates the roots and shows up above ground as leaf drop, sometimes with yellowing first and sometimes without much warning at all. Underwatering causes the same end result through the opposite mechanism, the tree shedding leaves to reduce how much water it needs to support. Checking soil moisture a few centimetres down, and confirming the pot or planting site actually drains, rules this cause in or out fast.
Sudden environmental change is the next most common cause, and the one most easily missed because it doesn't look like an obvious mistake. A citrus moved to a new spot, hit by an unexpected cold snap, or exposed to a sudden shift in light or wind after being sheltered, can respond by dropping a portion of its leaves as a stress response even when nothing else is wrong. This is usually self-limiting: the tree stabilises and holds its remaining leaves once conditions settle, without needing any direct treatment.
Sudden environmental change is the next most common cause, and the one most easily missed because it doesn't look like an obvious mistake.
Nutrient deficiency is worth checking if the dropped leaves showed yellowing beforehand, particularly a pattern of green veins against a yellowing leaf body, which points to iron or magnesium deficiency rather than a straightforward nitrogen shortage. Citrus are heavy feeders and do best on a citrus-specific fertiliser that includes these trace elements rather than a general all-purpose feed, applied through the main growing months rather than over winter when the tree is doing less.
Citrus leafminer is the pest-specific cause to rule out, especially on new growth: silvery, winding trails tunnelled through young leaves, sometimes followed by curling and premature drop of just that new flush, are distinctive enough that they're rarely mistaken for anything else once you've seen one. Established trees generally tolerate leafminer without major harm; the damage matters more on young trees still building their canopy.
Because so many of these causes look similar from a distance and only separate out once you check the right details, a stage-aware check-in, prompted at the point new growth flushes or conditions shift rather than left to a gardener's memory, catches most of this well before "a few dropped leaves" becomes "half the canopy gone."