Snails and slugs are the single most common pest complaint from New Zealand home gardeners, and the country's damp, mild climate is a big part of why. Both need moisture to move and feed, so regular rainfall, dew-heavy nights and a huge population of introduced garden snails with no serious natural predators here mean most gardens carry a bigger, more active population than a similarly-sized garden in a drier climate would.
Seedlings and young transplants take the worst of it. A snail or slug that would barely dent a mature plant's leaf can strip a week-old seedling overnight, which is why raising seedlings under some kind of cover until they've got a few true leaves is one of the most effective single things a beginner gardener can do. Lettuce, spinach, cabbage and other brassicas, strawberries and basil are all genuinely more exposed to slug and snail damage than most other crops, alongside general seedling damage across almost anything sown direct.
A snail or slug that would barely dent a mature plant's leaf can strip a week-old seedling overnight
Physical barriers have mixed evidence behind them. Copper tape gives snails and slugs a mild electric-style deterrent when their slime touches it, and it does work as a barrier around individual pots or raised bed edges, provided it's kept clean, since a build-up of dirt or old slime bridges the gap and stops it working. Crushed eggshell and diatomaceous earth are far more popular folk remedies than the evidence supports: neither reliably deters slugs, because their slime coating protects their soft bodies from sharp or abrasive surfaces, and diatomaceous earth in particular stops working the moment it gets wet, which in most New Zealand gardens is often.
Beer traps are a genuinely effective, low-cost option: a shallow dish of beer sunk to ground level draws slugs and snails in, and they drown in it. Empty and refill every couple of days, since it loses effectiveness once it goes flat or gets diluted by rain. Hand-picking at dusk or after rain, when slugs and snails are most active, is tedious but reliably reduces numbers, especially around vulnerable seedlings.
For a bigger population, bait is the practical option, but the two main types differ a lot in how safe they are around pets and other wildlife. Metaldehyde baits, the traditional option, are effective but genuinely toxic to dogs and cats if eaten, even in fairly small amounts. Iron-phosphate-based baits are the safer alternative for gardens with pets or that border reserves and streams: they break down into compounds already present in soil and are far less toxic to pets and birds at the quantities normally used, though they still shouldn't be left in easily-scoffed piles. Whichever you use, scatter it thinly rather than heaping it, and keep it away from areas pets or children regularly access.