Northland barely sees frost at all. Kaitaia's official climate record shows well under half a ground-frost day a month even at the coldest point of the year, and most home gardens further south of there see just as little. That single fact changes the whole shape of a Northland spring: there's no real "last frost date" to wait out, so the limiting factor is soil temperature and rainfall rather than cold.
That means Northland gardeners are often planting tomatoes, capsicums and courgettes by early September, a full six to eight weeks ahead of Waikato or Wellington. Lettuce, spinach, silverbeet, beetroot, carrots, peas and dwarf beans all go in just as early, alongside basil, coriander and the rest of the tender herbs that other regions have to hold back undercover.
The trade-off is humidity. Northland's warm, damp climate that makes early planting possible is the same climate that lets blight, powdery mildew and other fungal problems establish fast once summer humidity sets in. Space plants further apart than a Waikato or Canterbury guide would suggest, water at the base rather than overhead, and check the undersides of leaves regularly through summer rather than waiting for obvious damage.
The trade-off is humidity.
If there's a genuine risk in a Northland spring, it's planting too late rather than too early. A garden started in September has months of head start on pests and disease pressure that a November planting walks straight into at its worst.