Hi,
1. There is no option to do that. Even more so that the addresses of the user can be used for either shipping or billing.
What you can do though is to restrict your shipping methods to the country zone "Australia". That way, if the user selected a shipping address outside of Australia, he will get an error message on the selection of the shipping methods and won't be able to proceed unless he changes his shipping address to an address within Australia.
2. As point one, there is no option to do that as addresses are shared between billing and shipping. The best I can propose is to add a custom address field via the menu System>Custom field, do not require it, and add some CSS code to hide it for the billing address form.
3. That's not possible. It would require a complex customization of the code.
Note however that nothing is impossible here. It's just impossible with the options provided by default. But you can always contact a third party developer like our partners
www.hikashop.com/home/our-partners.html
and have that customized for you by them.