Tip - avoid completely.
The only way you are likely to save any money over buying in Australia is with a parallel import, which will leave you with no warranty, and may not even be a Hong Kong model. Your charge will not have the correct plug.
There is no duty in Hong Kong anyway, so buying duty free doesn't save you anything.
If you buy a new iPhone and iPad, then you are likely to have to pay GST on it when returning to Australia anyway, as they will likely be over your limit for exempt goods.
There are so many downsides - unless you are totally familiar with prices, where to get deals, and the exact equipment, then the potential pitfalls way out weigh any possible benefits.