Leaf curl means two genuinely different problems in a New Zealand garden, and mixing them up leads to the wrong treatment, since one is a fungal disease and the other is a pest.
Peach leaf curl is the classic case people usually mean when they say "leaf curl." It's a common fungal disease that specifically affects peach and nectarine trees, and it shows up in spring as leaves that turn reddish or purplish, thicken, pucker and distort dramatically compared to a healthy, flat leaf. Badly affected leaves often drop early, weakening the tree over the season if it happens year after year without treatment.
The standard, effective treatment for peach leaf curl is a copper-based fungicide, sprayed at two specific points in the tree's yearly cycle rather than once symptoms appear. The first spray goes on at bud swell, right at the very start of spring growth, before the fungus has a chance to infect the newly opening leaves. The second spray goes on again at leaf fall in autumn, to reduce the amount of fungal spore overwintering on the tree and bark ready to reinfect it the following spring. Spraying only after curled leaves are already visible is far less effective, since the fungus has already done its damage to that season's growth by the time symptoms show.
The first spray goes on at bud swell, right at the very start of spring growth, before the fungus has a chance to infect the newly opening leaves.
Leaf curling caused by aphids is a completely different problem, and it's a pest issue rather than a disease. Aphids cluster densely on soft new growth and feed by piercing plant cells, and a heavy colony can visibly curl and distort the leaves it's feeding on, which can look superficially similar to a diseased leaf at a glance. The key difference is what's actually on the leaf: aphid-curled leaves usually have the insects themselves clustered on the underside or in the curl, often with ants in attendance, whereas peach leaf curl shows no insects at all, just the leaf's own distorted, discoloured tissue. This site's dedicated aphid guide covers identification and non-chemical control, including hosing affected growth and encouraging ladybirds and hoverflies, in full detail, since the two problems and their treatments don't overlap.
Getting the diagnosis right matters because the treatments genuinely don't overlap. Copper fungicide sprayed at bud swell does nothing for an aphid infestation, and hosing off aphids does nothing for a fungal disease already established in a peach tree's leaf tissue. Check whether you're looking at a peach or nectarine with no insects present, which points to leaf curl, or another plant entirely with visible aphids clustered in the curl, which points to a pest problem, before deciding how to treat it.