What are the policies of OZ in relation to checking through on separate tickets?

There are three main possibilities at ICN:

1. They check your bags through to ZQN and are able to be checked in for your SYD-ZQN flight (note VA do not have OLCI for international flights).

2. They check your bgs though to ZQN only. You still need to check in with VA.

3. They check your bags through to SYD only.

In the first case you should be fine, simply using the airside transit security to go from arrivals to departures - but I doubt this will happen.

For the other two circumstances, you will need to go through immigration and customs (collecting any bags in the case of #3) and proceed to VA check-in. You would need an electronic Visa.