National Australia Bank was formed as National Commercial Banking Corporation of Australia Limited in 1982 by the merger of National Bank of Australasia and the Commercial Banking Company of Sydney. The resulting company was subsequently renamed National Australia Bank Limited. The expanded financial base of the merged entity triggered significant offshore expansion over ensuing years. Representative offices were established in Beijing (1982), Chicago (branch 1982), Dallas (1983), Seoul (1983, upgraded to a branch in 1990), San Francisco (1984), Kuala Lumpur (1984), Athens (1984, closed 1989), Frankfurt (1985, closed 1992), Atlanta (1986), Bangkok (1986), Taipei (1986 upgraded to branch 1990), Shanghai (1988, closed 1990), Houston (1989) and New Delhi (1989). In 1987, NAB bought Clydesdale Bank (Scotland) and Northern Bank (Northern Ireland and Republic of Ireland) from Midland Bank. It re-branded Northern Bank branches in the Republic of Ireland to National Irish Bank and changed both banks' logos from that of the Midland Bank. In 1990, NAB bought Yorkshire Bank (England and Wales).
Further acquisitions followed – Bank of New Zealand in 1992, which at the time had about a 26% market share in the New Zealand market, and Michigan National Bank (MNB) in 1995. NAB had earlier rationalised its operations in the US and closed its offices in Atlanta, Chicago, Dallas, Houston, and San Francisco in 1991. This period of rapid expansion through acquisition concluded with the purchases in 1997 of HomeSide Lending, a leading US mortgage originator and servicer based in Florida, and most significantly, the acquisition in 2000 of MLC Limited (and related MLC entities) for $4.56bn, one of the biggest mergers in Australian corporate history. NAB encountered a difficult period in the period 2000–2005. In 2000, NAB sold Michigan National Bank to ABN AMRO, then in 2001 sold HomeSide's operating assets for US$1.9b to Washington Mutual, the largest US savings and loan company, as well as the mortgage unit's loan-servicing technology and operating platform.

Recently, as part of a strategy to focus NAB on its domestic markets, the bank listed its US subsidiary Great Western Bank subsidiary on the New York Stock Exchange in October 2014 as part of an initial public offering. NAB sold its final 28.5% holding in Great Western in July 2015. In May 2015, NAB also confirmed it would de-merge its Clydesdale and Yorkshire Bank business in the UK, through an initial public offering. The business was partially floated on the London Stock Exchange and Australian Securities Exchange under a new holding company, CYBG plc, in February 2016, with the remaining shares distributed to NAB's shareholders.