Remember that Australia and New Zealand are in the Southern Hemisphere — seasons are flipped from what you might expect. Australia's climate also varies enormously by region. The tropical north (Queensland, Northern Territory, Western Australia's Kimberley) is best visited in the dry season from May to October, when humidity drops and wildlife congregates around shrinking water sources. The southern states — Victoria, New South Wales, South Australia, Tasmania — are most pleasant in summer, from December to February, though spring (September to November) and autumn (March to May) offer mild weather and fewer crowds.
New Zealand's peak season runs from December to February, with long days, warm temperatures, and everything open. It's a small country, but the weather can shift fast, so pack layers regardless of the season. Ski season runs from June to September, centered on the South Island's remarkable mountains and the North Island's volcanic slopes around Ruapehu. Shoulder months — October/November and March/April — are excellent for hiking, with fewer people on the tracks and still-reasonable weather.
The Pacific islands follow a tropical pattern. The dry season from May to October is the most comfortable time to visit Fiji, Tonga, Samoa, and most of Melanesia and Polynesia — lower humidity, less rain, and virtually no cyclone risk. The wet season from November to April brings higher temperatures, afternoon downpours, and the possibility of tropical cyclones. Travel is still possible in the wet season (and prices drop), but you should monitor weather forecasts and understand that some ferry and flight routes may be disrupted.
If you're timing your trip around specific experiences, a few dates matter. The Great Barrier Reef is best from June to October, when water visibility peaks and stinger season is over. Humpback whale migration along Australia's east coast runs from May to November, with peak viewing around Hervey Bay in August and September. French Polynesia's whale season is similar, July to October. New Zealand's Milford Track and other Great Walks book out months ahead for the peak summer season — plan early or aim for the shoulder months.