EXPLANATION The idea of having paragraphs is that each new paragraph signals a change of topic, so each paragraph should be about one topic only. That topic is usually clearly defined in one of the sentences in the paragraph, called the topic sentence. The rest of the paragraph should then be relevant to it.
In the example above, the topic sentence (in bold) asserts that Australians are not healthy. Read the paragraph keeping the topic sentence in mind and you will realise that the sentences in italics are not relevant to the topic that Australians are unfit. The last sentence is really part of another topic, namely the solution to the problem.
In the sample paragraph, the topic sentence is the first sentence. Such paragraphs are called open paragraphs. By contrast, we can build to the conclusion, placing the topic sentence at the end. Such paragraphs are periodic paragraphs (just as a period, a full stop, ends a sentence.) Below is an example of how the sample paragraph above might have been written with the topic sentence at the end.
Obesity is reaching epidemic proportions in Australia and associated medical problems are becoming more common. The incidence of diabetes is increasing and, amongst the adult population, we have the second-highest death rate from heart disease in the world. Perhaps Australians think that because we have so many sporting successes, we are a fit, sporting nation. We might well consider how many people actually take part in sport and how many people sit inactively on the sidelines. The myth of the fit, bronzed Aussie is just that, a myth. Australians are not as fit and healthy as most of us like to think we are.