Sure, you can do that.
Create a new zone called "domain2.com.au" and add each host you need to that zone.
If the zone hosts a mixture of public and private servers (servers on your LAN vs external servers) you might create individual zones for each host (e.g. a new forward lookup zone called "ecgw1.domain2.com.au"), that gets around having to create every single record you might want to resolve in your private version of "domain2.com.au".
Chris