Original: Underclothes, underclothing  and underwear are formal terms, while undergarments may be more casually referred to in Australia as Reg Grundys (rhyming slang for undies) or Reginalds, and in the UK as smalls (from the earlier smallclothes).
Simplified: In Australia, one may hear underwear being called "Reg Grundys" (rhyming slang for "undies") or "Reginalds", while in the UK the word "smalls" is sometimes used.