If you're worried about breaking the 20-year rule (and I believe this question does break that rule), you might want to ask the experts in the present-day social sciences over in /r/AskSocialScience. They have geographers, political scientists, and sociologists - all the right type of people to answer this question about how to define modern-day countries.

This is almost more of a political science question that a historical one, but the answer varies as you define what the countries listed are.

Nation is a group of people who share a common language, culture, or ethnicity. For example, the Kurdish people could be used as an example of a nation. In Canada, there is both the Aboriginal and Quebecois nations. A nation does not denote sovereignty.

A state is the sovereign entity over a specified geographic area. Pretty basic definition. The United States for example is a state, Iraq is a state, and Canada is a state.

Nation state implies that a nation of people has sovereignty over a specific geographic area. That nation also makes up the vast majority of the population of that state. Nation states tends to be smaller, as the larger a state is the more nations will have a larger chunk of their population.

So no. "melting pot" states are not nation states. Iceland or Egypt would be examples of a nation state, as the overwhelmingly majority of the population are from one nation.

Canada and Australia cannot be nation states due to the mixture of ethnic groups that make up it's population. For example, and simplicity, the Aboriginal and European nations that make up the populations of these countries.

The issue is, especially with the increase in Globalization, it is going to grow tougher to identify and label nation states, as various ethnic groups continue to move and mix around the globe.