Watermelon is a genuine crop in New Zealand's warmest regions and a marginal one almost everywhere else. It needs a long run of real summer heat to fill out and sweeten a fruit this size, and it needs room: this isn't a plant you tuck in between other crops and forget about. It's rated an intermediate crop for those two reasons together, not because any single step in growing it is difficult.
Where you garden decides your planting window. In Northland, Auckland, Bay of Plenty, Gisborne and Hawke's Bay, plant out from September through to November. In Waikato, Taranaki, Manawatu-Whanganui, Wellington, Nelson and Tasman, wait until October or November, once the soil has properly warmed. In Marlborough, the West Coast and Canterbury, hold off until November, and only attempt it in a hot, sheltered, north-facing spot with a full summer's heat still ahead of it. In Otago and Southland, it's not a realistic outdoor crop; without a greenhouse or tunnel to bank extra heat, the season isn't long or warm enough to bring fruit to ripeness.
Give each plant a full 2 metres of space. The vine sprawls rather than climbs, and squeezing it in among other crops just means wading through leaves to find the fruit later. Where space is tight, it can be trained up a strong trellis instead, but the vine itself can't carry the weight of a developing melon, so each fruit needs its own sling as it swells. It wants full sun for the whole day. Sweetcorn, beans and radish are good neighbours; keep it away from potatoes.
Give each plant a full 2 metres of space.
Water moderately but consistently while the fruit is sizing up, since irregular watering is a common cause of splitting. In the final week or two before harvest, ease off the watering: it's a widely used trick for concentrating sugar in the fruit rather than diluting it. Watermelon carries separate male and female flowers on the same vine and relies on bees to move pollen between them, so don't spray insecticide once flowering starts. Poor pollination, whether from too few bees around or the male and female flowers simply not opening on the same day, is the usual explanation when a vine flowers well but sets only small or oddly shaped fruit.
Powdery mildew is the main disease to watch for, especially in humid weather; water at the base of the plant rather than over the leaves to keep it down. Aphids and whitefly are the main pests, and both tend to cluster on the undersides of new leaves, so that's where to check first.
Expect 80 to 100 days from sowing to harvest, and only 1 to 3 fruit per plant: the vine puts its energy into a small number of large melons rather than many small ones. Don't rely on the calendar to tell you when to pick. Look for the tendril nearest the fruit's stem drying out and turning brown, and check the patch where the fruit rests on the ground: it should have turned from pale white to a creamy yellow. A ripe melon also gives a dull, hollow thud when you tap it, rather than a sharp slap, and the skin loses its shine. Watermelon doesn't keep ripening once it's off the vine, so it's worth waiting for these signs rather than guessing early.