No, you're not expected to know statistics when stating why the NHPA has been selected (unless you're accompanied by statistics in the question).
Last year I just remembered a few (maybe 4-5) general reasons as to why the conditions were chosen as an NHPA such as;
- Contributes largely to burden of disease (both mortality and morbidity)
- Contributes highly to health expenditure (in terms of direct, indirect and intangible costs)
- Has the potential for improved health outcomes.
- Highly preventable etc...
And whenever I got a question about why a specific condition has been chosen I would just shape my answer around what reasons I had remembered e.g. Cancer contributes to a high burden of disease, especially YLL because it is a terminating illness, which results in large number of premature deaths in Australia.
Another one could be; A large amount of money is spent on Asthma in a direct manner due to the costs of supplying inhalers, spacers and ambulance services for emergencies, burdening Australia's health expenditure.