The following table contains the by-elections that have occurred during the term of the 20th Parliament.
| Electorate and by-election | Date | Incumbent | Cause | Winner | |||
|---|---|---|---|---|---|---|---|
| Bruce | 1920 | 14 April | James Allen | Resignation | John Edie | ||
| Stratford | 1920 | 6 May | Robert Masters | Election declared void | Robert Masters | ||
| Bay of Plenty | 1920 | 30 September | William MacDonald | Death | Kenneth Williams | ||
| Patea | 1921 | 13 April | Walter Powdrell | Death | Edwin Dixon | ||
| Auckland East | 1921 | 2 November | Arthur Myers | Resignation | Clutha Mackenzie | ||
| Southern Maori | 1922 | 25 January | Hopere Uru | Death | Henare Uru | ||
| Dunedin North | 1922 | 21 June | Edward Kellett | Death | Jim Munro | ||