I can't see why you would not be able to use BD miles for VS flights to SYD.

However, be aware that using miles to fly with a Star Alliance carrier, such as Singapore Airlines, Thai Airways, Austrian Airlines, Air Canada, Air New Zealand, ANA or United Airlines could work out cheaper. Air New Zealand will also be introducing a premium economy product later this year:

Star Alliance awards from Zone 1 (UK, Ireland) to Zone 9 (Australia, NZ, South Pacific) costs

100,000 miles (economy) + taxes
125,000 miles (premium economy - where available) + taxes
150,000 miles (business - equivalent to Upper Class on Virgin) + taxes
200,000 miles (first class) + taxes

OR you can halve the number of miles and pay a small amount of money instead, meaning a flight would cost:

50,000 miles + £340 + taxes (economy)
62,500 miles + £425 + taxes (premium economy where available)
75,000 miles + £510 + taxes (business)
100,000 miles + £680 + taxes (first)

You cannot do miles+cash for Virgin flights.