Who ticketed it? VA or QR?

QR isn't listed as a codeshare/partner airline with VA nor are they listed as an interline/through-check airline. https://www.virginaustralia.com/au/e...line-partners/

You can still ask them to through check your bag at CNS and see how you go, if it is on the same ticket it might still be possible. If you can check your baggage through that would be a big help, because you'll need to transfer terminals at SYD and doing it with baggage is a pain. You definitely won't be able to get your QR tickets at CNS, but that isn't too big a drama.

6 hours should be plenty to get into SYD. Just take the train to Circular Quay and have a look around. There are lots of posts about this in the forum already, quite a common question.