If one believes the SMH, Macquarie Bank is having enough trouble with its infrastructure subsidiaries at the moment that it could barely afford to re-pave the disabled car park, let alone throw down another strip. They will:
a. squeeze everything they can out of YSSY without spending... you think BAA running EGLL into the ground was bad...;
b. use their pre-emption right to halt any development of a 'major' airport within 100km of YSSY, so as to maximise revenue flowing through that asset.
Hence it has to be Willy or CBR, both of which are >100km from YSSY.