Virgin Australia's web site says clearly that you do have access:
Delta partnership on va.com.au:
SkyMiles Medallion Diamond, Platinum and Gold members receive complimentary entry into the Virgin Australia lounge network*.
* On the same day of travel only.
I had no trouble accessing DJ lounges (in domestic terminals with a DJ flight) as a Sky Club member, but can't (yet) speak from personal experience as a Delta Gold.