There is absolutely no way that a VA Platinum will be able to enter the Qantas F Lounge travelling Qatar Airways in Economy or Business.
Velocity Gold and Platinum members flying ticketed Economy will likely gain access to the 'designated' lounge at each airport – at Sydney, that's Air New Zealand and in Melbourne, that's Marhaba.
Those flying ticketed Business or First can access the Qantas Business/First lounges respectively because of class of travel, not elite status.
Access to the Qantas F Lounge as a QR Platinum is a oneworld benefit, which does not apply to the Velocity partnership.
In Doha, VA elite access would be the Gold or Platinum Lounges. Al Mourjan is Business ticketed only and Al Safa is First ticketed only, except for Qatar's own Platinum Privilege Club members travelling in ticketed Business. That won't extend to Velocity Platinums.