The agent is correct. You have to pick-up your bags at first port of entry into Australia (same as with the US) so you'll collect your bags in SYD and can choose to simply walk out of the building instead of going left to the VA re-check counter.

DL does not operate the flight from SYD - MEL (VA does) and there could be a myriad of reasons why you never ultimately make your SYD - MEL flight -- no one is going to call and ask if you don't show up (and if they do I'm sure you can come up with a valid reason on the day of that you didn't make it from the international to the domestic terminal).

It is very unlikely DL will care that you don't make the flight (or that they will ever know since it is an award ticket so I don't know that VA is reporting anything back to DL because there is no mileage credit at play).

I have not done this explicitly on my travels from the US to MEL (via SYD) but I feel 100% confident I could have I'd wanted to. I would feel confident in proceeding as planned