Your verb form depends on whether you think of Westpac as a group of people running a company (BrE) or as as single-person legal fiction (AmE). "Westpac" is not a pronoun. It's a name, so it's a proper noun.
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Westpac is not speaking on its own behalf. If it were, the statement would say something like "We, the Westpac Board of Directors, are pleased to announce that we have not lost any clients.Westpac has not lost any clients