I was just reading the Crikey list of Australian politicians who’ve
changed parties. While I would be the last person to play down the
differences between the Liberal and National parties, they are at least
notionally on the same side of politics. Most of the changes cited are
from Liberal to National or vice versa, or from major parties to minor
or even fringe parties (sometimes parties of one).

Relatively few are
full floor-crossings from one party that can reasonably expect to form a
government at some time to an opposing party that has similarly
realistic expectations. Billy Hughes is the major exception quoted, but
there are notable others:

    • A number of ALP politicians crossed the floor with Billy Hughes during
      WWI, the most notable of whom was George Pearce, who became a long-time
      non-Labor (Nationalist and United Australia) Minister and Australia’s
      longest-serving Senator, topping 30 years.
    • A number of ALP politicians crossed the floor during the
      Depression years of the Scullin Government, the most notable of whom
      was Joe Lyons, the future United Australia PM. His colleague JA Guy
      also became a United Australia and Liberal MP, and later Senator.
    • One defector (although to a party of the same notional side of
      politics) who actually brought down a Government was “Stabber Jack”
      Beasley, who led his Lang Labor colleagues (including Eddie Ward and
      Bert James)
      across the floor to support Lyons’ no-confidence motion in the Scullin
      Government. Beasley was later re-admitted to the ALP after Lang’s influence
    • At the State level in Tasmania, Carroll Bramich crossed the floor from
      Labor to Liberal in the mid-1950s, but instead of calling on the
      leader (Rex Townley) to form a government, the Governor agreed to
      Cosgrove’s request for an election and the ALP was re-elected. Bramich
      was also re-elected as a Liberal MHA.

      I can’t recall an Australian example after WWI of an MP
      from a major non-Labor
      party crossing the floor to join the ALP. It has happened in the UK,
      with one example being the notorious Oswald Mosley: Conservative to
      Independent to Labour (Cabinet Minister) to New Party to (out of
      Parliament) British Union of Fascists.

      Winston Churchill, of course, crossed from the Conservatives to the governing
      Liberals before Labour was a major parliamentary force, but never actually
      crossed the floor back, with his reversal of allegiance coming after he had
      lost his seat in 1922.

      David Lloyd George’s son Gwilym and daughter Megan
      were both elected as Liberal MPs but later changed parties and were
      elected: Gwilym as a Tory and Megan for Labour. However, the Liberal Party
      had already declined to minor party status before either of them entered Parliament.