A "last frost date" is simply the point in the season after which frost becomes unlikely enough in a given area that it's reasonably safe to plant tender crops outdoors. It's not a guarantee, just a shift in the odds, and exactly when that point falls varies significantly across New Zealand.
That variation is why this site's own planting guidance is built around broad climate zones rather than a single national date. Warmer northern regions, Northland, Auckland, Bay of Plenty, Gisborne and Hawke's Bay, lose their frost risk earliest in spring, so tender crops can go in from September in those areas. The more temperate middle band, Waikato, Taranaki, Manawatū-Whanganui, Wellington, Nelson and Tasman, generally needs to wait until October. Marlborough, the West Coast and Canterbury run cooler again and are usually safest from November, while Otago and Southland, the coolest zone, often don't clear frost risk until later still, sometimes not at all for the most frost-tender crops in a given season. This mirrors the same regional pattern behind the planting-month guidance elsewhere on this site: the same crops planted in Auckland in September are typically not planted in Otago until well into early summer.
Tender summer crops like tomatoes, capsicums, zucchini, cucumber and basil are frost-tender, meaning a hard frost can kill or badly damage them outright. That's why the standard advice for all of them is to plant out once the last frost risk has genuinely passed for your area, rather than on a fixed calendar date that ignores how different one region's spring actually is from another's. A warm, sunny week in early spring doesn't mean frost is finished. Late frosts happen well after a mild spell has convinced people the cold is over, especially further south and inland.
The risk of planting too early is real and immediate. Tender seedlings have very little tolerance for cold: a single frosty night can blacken and kill young tomato, capsicum or zucchini plants that looked perfectly healthy the day before, undoing weeks of raising them from seed in a single overnight cold snap. Losing a tray of seedlings this way is one of the most common and most avoidable setbacks a home gardener faces, and it's almost always the result of planting by the calendar rather than by actual conditions.
If you're not sure whether your own area's frost risk has passed, the safer approach is to wait a little longer rather than gamble on an early planting date. A tender crop planted a week or two later than the earliest possible date loses very little; a tender crop planted into a late frost can lose the entire planting outright.
A tender crop planted a week or two later than the earliest possible date loses very little; a tender crop planted into a late frost can lose the entire planting outright.