Hmm...back in the days when NW operated the route themselves they used to have a deal where they doubled the airmiles on any trip that included Sydney.
The accountants had done the math based on their US customers, but if you started your trip in London and did LHR-MSP-LAX-SYD-LAX-MSP-LHR then you picked up enough miles for a free LHR-SYD return every second trip.