Population of New Zealand is nearing 4.5 million - about the same size as Sydney which is about 4.2 million.The Australian pop is 21 million.
The indigenous population in Australia is Melanesian, while New Zealand Maori are Polynesian.
Australia is huge with long distances between places.Their countryside has much less variety than New Zealand. There are attractive areas, but the scenery in New Zealand is much more varied and beautiful.
Many of the immigrants in Australia were originally from Italy, Lebanon, Greece, and more recently Vietnam and other parts of Asia.These days they are getting a lot more Middle Eastern and African etc.Although it really does depend where you live.
New Zealand immigrants are more from the Pacific Islands, with only small pockets from Italy, Dalmatia, etc.Our main immigrants these days are from Asia - Korea, Taiwan, China, Japan, etc, South Africa.
So we have rather different immigrant population mixes which in turn changes the cultures.Although there are many of the same people - the numbers are different for the different races.
The governing systems are quite different.Australia has a government in each State as well as a Federal government.
New Zealand has one government with an MMP system.
Australia has a lot of natural resources - minerals etc so work in the mines is extremely well paid.Perth and Western Australia is seeing a huge boom due to the mining.Perth housing is now the most expensive in Australia. This has a down side in that it is difficult to get the young people to aspire to jobs with much poorer pay yet very important.Health, Education, shops, offices etc.
Australia has long term drought in many areas.NZ has drought but it does not have the same impact as it has in Australia and the drought is usually short lived.We get a lot of rain in most parts of the country.
Their incomes generally are said to be about 30% higher than in New Zealand.Both countries have a major shortage of medical staff, teachers and many service people and tradespeople. NZ'ers tend to immigrate to Australia to get better salaries and still be close to home.Australians go to USA and UK for the same reasons.
Schooling in Australia is variable from state to State.A lot of people put their children into private schools there.Not so much in New Zealand. I think rather than the country the city or where you live can make a difference schooling wise, despite a national curriculum in New Zealand.
I think Australians are probably a little more nationalistic than NZ'ers.There is a lot of competition between the 2 countries.
Especially with sport.But in the end we are still close neighbours, friends and will support each other if necessary.
Australians tend to be generally more brash than New Zealander's. More American.
The climate in both countries varies, but the northern part opf Australia is tropical to temperate in the south.NZ is subtropical in the north to temperate.
Australia has snakes- some poisonous, and poisonous spiders, while NZ does not.
Their native animals are very different to those of New Zealand.
So I hope that tells you some differences!