Australia and New Zealand are often combined into a single Southern Hemisphere itinerary, but the countries are so different in character that treating them interchangeably leads to under-appreciating both. They reward different types of travelers.
Australia's major cities — Sydney, Melbourne, Brisbane — are sophisticated, multicultural, food-obsessed cities that happen to have extraordinary nearby natural environments. Sydney's harbor setting and beach culture give it an outdoor atmosphere unusual in a city of 5 million. Melbourne's coffee, restaurant, and cultural scene is among the best in the Asia-Pacific region. Both are expensive by global standards: $200–$300/day per person is realistic for comfortable travel.
New Zealand offers something fundamentally different: the combination of dramatic natural landscape with a small, culturally interesting, English-speaking country that's easy to navigate. The landscape ranges from fjords (Fiordland) to geothermal zones (Rotorua) to alpine mountains (Southern Alps) to volcanic coastlines — more geographical diversity per square kilometer than almost anywhere on earth. Wellington, the capital, has a food and arts scene that punches far above its size.
Decision framework: if you want an urban experience with excellent food and culture plus nearby beaches and nature, prioritize Sydney and Melbourne. If you want landscape-first travel with outdoor activities (hiking, skiing, kayaking) as the primary draw, prioritize New Zealand with its Queenstown, Milford Sound, and Tongariro Alpine Crossing experiences.
Best combination if visiting both: fly into Sydney, take the Pacific Coast Highway south through Melbourne, then fly Auckland–Queenstown–Milford Sound, returning from Christchurch or Auckland. The reverse works equally well.