By-elections to the Wellington City Council occur to fill vacant seats in the City Council. The death, resignation, bankruptcy or expulsion of a sitting councillor can cause a by-election to occur. By-elections were particularly frequent in the 1980s and 1990s but have become a rarity in recent years with only three occurring since 2000.

Local by-elections normally have lower turnouts than full local body elections. A noted case occurred in 1960 when a vacancy triggered a by-election which had a turnout of only 7.7% of voters, prompting Mayor Frank Kitts to label the light turnout a "deplorable state of affairs" as "...there were responsible and worthwhile citizens who were prepared to devote their time and money in service to the people".^[1] By-elections on the city council were on occasion deferred if a substantial majority of the council agreed to fill the vacancy by appointment, resulting in the highest polling unsuccessful candidate at the previous election being appointed to the council unless there is a public demand for a poll to be held (known as extraordinary vacancies). This arrangement has happened several times such as in 1973 when Olive Smuts-Kennedy resigned her seat and Seton Nossiter was appointed to replace her, in 1979 when Irvine Yardley resigned and was replaced with Tala Cleverley after several higher polling candidates declined the appointment,^[2] and in 1985 when Leone Harkness resigned and her council seat which was taken by Bruce Harris.^[3]

Two mayors first entered the Council via by-elections; John Luke (in 1905) and Celia Wade-Brown (in 1994).