Wellington's frost season is short. The city's own climate record shows ground frost easing by August, one of the earliest clearances in the country, and averaging under one and a half days a month even at the coldest point of winter. On frost risk alone, Wellington gardeners could reasonably start planting tender crops earlier than almost anywhere else in the lower North Island.
Wind is the reason most don't. A Wellington spring gale, particularly a southerly straight off the strait, does more damage to an unstaked tomato or bean plant in one afternoon than a light frost would do in a week, and it's a far more reliable feature of a Wellington spring than cold ever is. Stake tall crops the day they go in the ground, not once they've grown tall enough to need it, and choose the most sheltered spot available: against a north-facing fence or wall rather than out in the open.
With shelter sorted, the planting itself is straightforward: lettuce, spinach, silverbeet, peas, beetroot, carrots and dwarf beans from September, tomatoes, capsicums and courgette following from October. Basil and other frost-tender herbs are usually safe by October here too, earlier than Waikato or Canterbury gardeners can manage, since Wellington's frost risk clears so much sooner than either.
With shelter sorted, the planting itself is straightforward: lettuce, spinach, silverbeet, peas, beetroot, carrots and dwarf beans from September, tomatoes, capsicums and courgette following from October.