SYDNEY, Nov 20 (Reuters) - Westpac Banking Corp , Australia's third largest bank by market capitalisation, said on Monday it would raise A$750 million ($488.10 million) in an Additional Tier 1 (AT1) capital transaction. ($1 = 1.5366 Australian dollars) (Reporting by Scott Murdoch in Sydney; Editing by Muralikumar Anantharaman)