It's easy to assume Waikato follows Auckland's calendar simply because of how close the two regions sit, but the data says otherwise. Hamilton averages over eleven ground-frost days in July alone, more than five times Auckland's figure for the same month, and that frost risk typically doesn't clear until November. Inland, low-lying Waikato holds cold air on clear nights in a way the coast simply doesn't.
That gap matters most for tender crops. Tomatoes, capsicums, courgettes and cucumber planted in Waikato on an Auckland timeline, early September, run a real risk of getting hit by a frost that Auckland gardeners never see that year. Wait until October at the earliest, and later inland or in low frost-prone spots.
The rest of the spring list isn't frost-sensitive and can go in earlier regardless: lettuce, spinach, silverbeet, peas, beetroot, carrots and dwarf beans are all safe from September on. Once summer sets in, Waikato's humid, relatively still air, less coastal breeze than Auckland or the Bay of Plenty, means airflow around plants is worth planning for from the start rather than fixing after the fact. Space crops a little wider than a bag of seed suggests and stake tomatoes early so foliage doesn't end up in a dense, damp tangle by January.
The rest of the spring list isn't frost-sensitive and can go in earlier regardless: lettuce, spinach, silverbeet, peas, beetroot, carrots and dwarf beans are all safe from September on.