So you've flown Virgin twice in the last 11 months ... have you thought about whether retaining Platinum is worth the effort? You'll still have a soft landing down to Gold, which gives you many/most of the Platinum benefits.
If you want to retain Platinum, you're looking at one or two multi-sector return trips in Business class. From Melbourne, look at fares to North Queensland -- Townsville, Proserpine, Cairns, etc, flying via Sydney or Brisbane. Flying to Gold Coast via Canberra or Brisbane via Sydney would also give you large numbers of Status Credits.