Summer (December–February): the busiest, most reliable window
Both islands are at their most accessible in summer — every alpine pass and hiking track is open, days are long, and it's the best season for both Bay of Islands beach time and South Island glacier and fjord access. It's also peak season: expect the highest prices and busiest roads, especially around Queenstown and Milford Sound, and over the Christmas–New Year period when New Zealanders themselves are on their own summer holidays.
Autumn (March–May): the shoulder-season sweet spot
Many experienced visitors consider this the best all-round window — settled weather, thinner crowds than summer, and lower prices, while most tracks and passes remain open into April. The South Island's beech forests turn gold in April, an underrated season for scenic driving.
Winter (June–August): a genuinely different South Island
This is when the South Island's ski season runs, centred on Queenstown and Wanaka — a real reason to visit that doesn't exist the rest of the year. Some high alpine passes can close temporarily after snowfall, so South Island winter road trips need more flexible planning than a summer one. The North Island's winter, by contrast, is mild: Auckland and the Bay of Islands rarely see anything close to freezing at sea level, making the North a comfortable winter destination for visitors who aren't chasing snow.
Rotorua and Wellington in winter
Rotorua's geothermal attractions run year-round regardless of season — steam and hot pools are, if anything, more atmospheric on a cold morning. Wellington's museums and indoor food scene also make it a sensible winter stop, though the city's famous wind is sharper in the colder months.
Spring (September–November): unpredictable but green
Spring can be changeable, especially on the South Island where late-season snow is still possible into early spring at higher elevations. It's a quieter, greener season on both islands, with fewer crowds than summer but less certainty about track and pass conditions than autumn.
The short version
If flexibility matters more than budget, summer is the safest all-round choice for either island. If avoiding crowds matters more, autumn offers nearly the same access with better prices. Winter is really a South Island decision — go if skiing is the point, otherwise the North Island's mild winter is the more comfortable choice.