Here in the US, we are accustomed to the concept of a national practice for our Big 4 firms. Although each of the local offices, in each city, in each state has a Managing Partner and local partners, the firms are rarely separate legal entities. A partner is a partner of the National firm, not of the local Chicago office, for example, for legal purposes.
From The Press in New Zealand:
Ernst & Young is thought to have driven the merger with rival Christchurch accountancy firm KPMG in a bid to build itself up in the local market.