Converting to BA is something I would avoid since BA's idea of redemptions is charging a LARGE amount of money in fuel surcharges

According to http://www.marriott.com.au/rewards/usepoints/morerew.mi

125,000 Marriott Miles will net 50,000 US Airways or Alaska Miles
30,000 Marriott Miles will net 10,000 US Airways or Alaska Miles

Given 110k US miles needed for business class r/t to AU/NZ, a total of 280k marriott miles needed for a business return on Star Alliance
The same applies using Alaska Miles for business r/t on Qantas

Assuming that AS doesn't charge fuel surcharges, I would use one of these plans depending on which airline can be found for availability to redeem on

The transfer rate to United Airlines is the same as that to US Airways, however the redemption cost on United is 135k rather than the 110k that US Airways charges, so would use US rather than UA

Delta could be an option if neither Qantas nor Star have availability, but the redemption cost is higher again at a minimum of 150k Delta Miles for a r/t business class ticket ( which is more than a 1st class award using AS or US miles )