I am wanting to register a domain, however, I am not sure if I am allowed. The domain I want to register is already registered as a .com, however, say I want to register the same domain just as a .net - am I allowed to do that?

Or a domain registered as .com, .ca, .co.nz etc. am I allowed to register one say .com.au?

The website is going to be very similar to the .com - just specialized for Australia.